FA-76671 / Email MIME structure / Open access

Reassemble RFC 2231 style filename parameter continuations: single extended precedence · case 01

Continuation segments override a complete filename* value.

Verified by executionVariant 1 · 8 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The single extended precedence decision evaluates `if single is not None and 0 not in segs:` where the contract requires `if single is not None:`.

VERIFIED REPAIR

Use `if single is not None:` for the single extended precedence decision and keep every other rule of the model unchanged.

Unsuccessful approach: Yielding to the plain filename still lets another source override filename*. The attempted `if single is not None and plain is None:` still disagrees with a fixture.

Case contract

params are [name, value] pairs; names are case-insensitive. "filename*" (a single extended value charset'lang'text) wins over everything. Otherwise segments "filename*<n>" (plain) and "filename*<n>*" (percent-encoded) with decimal n without leading zeros ("0" allowed) are joined in numeric order from 0, stopping at the first gap; only an encoded segment 0 carries the charset'lang' prefix. Bytes of all segments are concatenated before decoding with the charset (utf-8, iso-8859-1 or us-ascii; others decode as utf-8; undecodable bytes become U+FFFD). Without a segment 0 the plain "filename" value (or None) is returned.

Why this case matters

Long or non-ASCII attachment names are split into continuations; reassembly errors garble or truncate names.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
from urllib.parse import unquote_to_bytes
N = 1
observations = []
def solve(params):
    single = None
    plain = None
    segs = {}
    for name, value in params:
        n = name.lower()
        if n == 'filename*':
            single = value
        elif n == 'filename':
            plain = value
        elif n.startswith('filename*'):
            rest = n[9:]
            enc = rest.endswith('*')
            num = rest[:-1] if enc else rest
            if num.isdigit() and (num == '0' or not num.startswith('0')):
                segs[int(num)] = (enc, value)
    def codec(cs):
        cs = (cs or 'us-ascii').lower()
        return cs if cs in ('utf-8', 'iso-8859-1', 'us-ascii') else 'utf-8'
    if single is not None and 0 not in segs:
        cs, _, rest = single.partition("'")
        _, _, value = rest.partition("'")
        return unquote_to_bytes(value).decode(codec(cs), 'replace')
    if 0 not in segs:
        return plain
    charset = 'us-ascii'
    chunks = []
    expect = 0
    for i in sorted(segs):
        if i != expect:
            break
        enc, v = segs[i]
        if enc and i == 0:
            charset, _, rest = v.partition("'")
            _, _, v = rest.partition("'")
        chunks.append(unquote_to_bytes(v) if enc else v.encode('latin-1', 'replace'))
        expect += 1
    return b''.join(chunks).decode(codec(charset), 'replace')
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('twelve segments', ([['filename*0', 'p0.'], ['filename*1', 'p1.'], ['filename*10', 'p10.'], ['filename*11', 'p11.'], ['filename*2', 'p2.'], ['filename*3', 'p3.'], ['filename*4', 'p4.'], ['filename*5', 'p5.'], ['filename*6', 'p6.'], ['filename*7', 'p7.'], ['filename*8', 'p8.'], ['filename*9', 'p9.'], ['filename', 'short1']],), 'p0.p1.p2.p3.p4.p5.p6.p7.p8.p9.p10.p11.'), ('gap in segments', ([['filename*0', 'a1'], ['filename*1', 'b'], ['filename*3', 'd']],), 'a1b'), ('leading zero index', ([['filename*0', 'a'], ['filename*1', 'b1'], ['filename*01', 'X']],), 'ab1'), ('charset only on first segment', ([['filename*0*', "utf-8''caf%C3"], ['filename*1*', '%A9_1.txt']],), 'café_1.txt'), ('quote inside plain first segment', ([['filename*0', "it's"], ['filename*1', 'fine1']],), "it'sfine1"), ('single extended value wins', ([['filename', 'plain.txt'], ['filename*', "utf-8''na%C3%AFve1.txt"], ['filename*0', 'seg']],), 'naïve1.txt'), ('orphan continuation', ([['filename', 'plain1.txt'], ['filename*1', 'orphan']],), 'plain1.txt'), ('upper-case names', ([['FileName*0', 'up'], ['FILENAME*1', 'per1']],), 'upper1')], 2: [('twelve segments', ([['filename*0', 'p0.'], ['filename*1', 'p1.'], ['filename*10', 'p10.'], ['filename*11', 'p11.'], ['filename*2', 'p2.'], ['filename*3', 'p3.'], ['filename*4', 'p4.'], ['filename*5', 'p5.'], ['filename*6', 'p6.'], ['filename*7', 'p7.'], ['filename*8', 'p8.'], ['filename*9', 'p9.'], ['filename', 'short2']],), 'p0.p1.p2.p3.p4.p5.p6.p7.p8.p9.p10.p11.'), ('gap in segments', ([['filename*0', 'a2'], ['filename*1', 'b'], ['filename*3', 'd']],), 'a2b'), ('leading zero index', ([['filename*0', 'a'], ['filename*1', 'b2'], ['filename*01', 'X']],), 'ab2'), ('charset only on first segment', ([['filename*0*', "utf-8''caf%C3"], ['filename*1*', '%A9_2.txt']],), 'café_2.txt'), ('quote inside plain first segment', ([['filename*0', "it's"], ['filename*1', 'fine2']],), "it'sfine2"), ('single extended value wins', ([['filename', 'plain.txt'], ['filename*', "utf-8''na%C3%AFve2.txt"], ['filename*0', 'seg']],), 'naïve2.txt'), ('orphan continuation', ([['filename', 'plain2.txt'], ['filename*1', 'orphan']],), 'plain2.txt'), ('upper-case names', ([['FileName*0', 'up'], ['FILENAME*1', 'per2']],), 'upper2')], 3: [('twelve segments', ([['filename*0', 'p0.'], ['filename*1', 'p1.'], ['filename*10', 'p10.'], ['filename*11', 'p11.'], ['filename*2', 'p2.'], ['filename*3', 'p3.'], ['filename*4', 'p4.'], ['filename*5', 'p5.'], ['filename*6', 'p6.'], ['filename*7', 'p7.'], ['filename*8', 'p8.'], ['filename*9', 'p9.'], ['filename', 'short3']],), 'p0.p1.p2.p3.p4.p5.p6.p7.p8.p9.p10.p11.'), ('gap in segments', ([['filename*0', 'a3'], ['filename*1', 'b'], ['filename*3', 'd']],), 'a3b'), ('leading zero index', ([['filename*0', 'a'], ['filename*1', 'b3'], ['filename*01', 'X']],), 'ab3'), ('charset only on first segment', ([['filename*0*', "utf-8''caf%C3"], ['filename*1*', '%A9_3.txt']],), 'café_3.txt'), ('quote inside plain first segment', ([['filename*0', "it's"], ['filename*1', 'fine3']],), "it'sfine3"), ('single extended value wins', ([['filename', 'plain.txt'], ['filename*', "utf-8''na%C3%AFve3.txt"], ['filename*0', 'seg']],), 'naïve3.txt'), ('orphan continuation', ([['filename', 'plain3.txt'], ['filename*1', 'orphan']],), 'plain3.txt'), ('upper-case names', ([['FileName*0', 'up'], ['FILENAME*1', 'per3']],), 'upper3')], 4: [('twelve segments', ([['filename*0', 'p0.'], ['filename*1', 'p1.'], ['filename*10', 'p10.'], ['filename*11', 'p11.'], ['filename*2', 'p2.'], ['filename*3', 'p3.'], ['filename*4', 'p4.'], ['filename*5', 'p5.'], ['filename*6', 'p6.'], ['filename*7', 'p7.'], ['filename*8', 'p8.'], ['filename*9', 'p9.'], ['filename', 'short4']],), 'p0.p1.p2.p3.p4.p5.p6.p7.p8.p9.p10.p11.'), ('gap in segments', ([['filename*0', 'a4'], ['filename*1', 'b'], ['filename*3', 'd']],), 'a4b'), ('leading zero index', ([['filename*0', 'a'], ['filename*1', 'b4'], ['filename*01', 'X']],), 'ab4'), ('charset only on first segment', ([['filename*0*', "utf-8''caf%C3"], ['filename*1*', '%A9_4.txt']],), 'café_4.txt'), ('quote inside plain first segment', ([['filename*0', "it's"], ['filename*1', 'fine4']],), "it'sfine4"), ('single extended value wins', ([['filename', 'plain.txt'], ['filename*', "utf-8''na%C3%AFve4.txt"], ['filename*0', 'seg']],), 'naïve4.txt'), ('orphan continuation', ([['filename', 'plain4.txt'], ['filename*1', 'orphan']],), 'plain4.txt'), ('upper-case names', ([['FileName*0', 'up'], ['FILENAME*1', 'per4']],), 'upper4')], 5: [('twelve segments', ([['filename*0', 'p0.'], ['filename*1', 'p1.'], ['filename*10', 'p10.'], ['filename*11', 'p11.'], ['filename*2', 'p2.'], ['filename*3', 'p3.'], ['filename*4', 'p4.'], ['filename*5', 'p5.'], ['filename*6', 'p6.'], ['filename*7', 'p7.'], ['filename*8', 'p8.'], ['filename*9', 'p9.'], ['filename', 'short5']],), 'p0.p1.p2.p3.p4.p5.p6.p7.p8.p9.p10.p11.'), ('gap in segments', ([['filename*0', 'a5'], ['filename*1', 'b'], ['filename*3', 'd']],), 'a5b'), ('leading zero index', ([['filename*0', 'a'], ['filename*1', 'b5'], ['filename*01', 'X']],), 'ab5'), ('charset only on first segment', ([['filename*0*', "utf-8''caf%C3"], ['filename*1*', '%A9_5.txt']],), 'café_5.txt'), ('quote inside plain first segment', ([['filename*0', "it's"], ['filename*1', 'fine5']],), "it'sfine5"), ('single extended value wins', ([['filename', 'plain.txt'], ['filename*', "utf-8''na%C3%AFve5.txt"], ['filename*0', 'seg']],), 'naïve5.txt'), ('orphan continuation', ([['filename', 'plain5.txt'], ['filename*1', 'orphan']],), 'plain5.txt'), ('upper-case names', ([['FileName*0', 'up'], ['FILENAME*1', 'per5']],), 'upper5')]}
for _label, _args, _expected in _CASES[N]:
    check(_label, solve(*_args), _expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
twelve segments	`p0.p1.p2.p3.p4.p5.p6.p7.p8.p9.p10.p11.`	`p0.p1.p2.p3.p4.p5.p6.p7.p8.p9.p10.p11.`	Passed
gap in segments	`a1b`	`a1b`	Passed
leading zero index	`ab1`	`ab1`	Passed
charset only on first segment	`café_1.txt`	`café_1.txt`	Passed
quote inside plain first segment	`it'sfine1`	`it'sfine1`	Passed
single extended value wins	`seg`	`naïve1.txt`	Failed
orphan continuation	`plain1.txt`	`plain1.txt`	Passed
upper-case names	`upper1`	`upper1`	Passed

SHA-256 / af9eed93117695b5016aba81feada92b6e9243245063e233674e059408cd0286

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
from urllib.parse import unquote_to_bytes
N = 1
observations = []
def solve(params):
    single = None
    plain = None
    segs = {}
    for name, value in params:
        n = name.lower()
        if n == 'filename*':
            single = value
        elif n == 'filename':
            plain = value
        elif n.startswith('filename*'):
            rest = n[9:]
            enc = rest.endswith('*')
            num = rest[:-1] if enc else rest
            if num.isdigit() and (num == '0' or not num.startswith('0')):
                segs[int(num)] = (enc, value)
    def codec(cs):
        cs = (cs or 'us-ascii').lower()
        return cs if cs in ('utf-8', 'iso-8859-1', 'us-ascii') else 'utf-8'
    if single is not None and plain is None:
        cs, _, rest = single.partition("'")
        _, _, value = rest.partition("'")
        return unquote_to_bytes(value).decode(codec(cs), 'replace')
    if 0 not in segs:
        return plain
    charset = 'us-ascii'
    chunks = []
    expect = 0
    for i in sorted(segs):
        if i != expect:
            break
        enc, v = segs[i]
        if enc and i == 0:
            charset, _, rest = v.partition("'")
            _, _, v = rest.partition("'")
        chunks.append(unquote_to_bytes(v) if enc else v.encode('latin-1', 'replace'))
        expect += 1
    return b''.join(chunks).decode(codec(charset), 'replace')
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('twelve segments', ([['filename*0', 'p0.'], ['filename*1', 'p1.'], ['filename*10', 'p10.'], ['filename*11', 'p11.'], ['filename*2', 'p2.'], ['filename*3', 'p3.'], ['filename*4', 'p4.'], ['filename*5', 'p5.'], ['filename*6', 'p6.'], ['filename*7', 'p7.'], ['filename*8', 'p8.'], ['filename*9', 'p9.'], ['filename', 'short1']],), 'p0.p1.p2.p3.p4.p5.p6.p7.p8.p9.p10.p11.'), ('gap in segments', ([['filename*0', 'a1'], ['filename*1', 'b'], ['filename*3', 'd']],), 'a1b'), ('leading zero index', ([['filename*0', 'a'], ['filename*1', 'b1'], ['filename*01', 'X']],), 'ab1'), ('charset only on first segment', ([['filename*0*', "utf-8''caf%C3"], ['filename*1*', '%A9_1.txt']],), 'café_1.txt'), ('quote inside plain first segment', ([['filename*0', "it's"], ['filename*1', 'fine1']],), "it'sfine1"), ('single extended value wins', ([['filename', 'plain.txt'], ['filename*', "utf-8''na%C3%AFve1.txt"], ['filename*0', 'seg']],), 'naïve1.txt'), ('orphan continuation', ([['filename', 'plain1.txt'], ['filename*1', 'orphan']],), 'plain1.txt'), ('upper-case names', ([['FileName*0', 'up'], ['FILENAME*1', 'per1']],), 'upper1')], 2: [('twelve segments', ([['filename*0', 'p0.'], ['filename*1', 'p1.'], ['filename*10', 'p10.'], ['filename*11', 'p11.'], ['filename*2', 'p2.'], ['filename*3', 'p3.'], ['filename*4', 'p4.'], ['filename*5', 'p5.'], ['filename*6', 'p6.'], ['filename*7', 'p7.'], ['filename*8', 'p8.'], ['filename*9', 'p9.'], ['filename', 'short2']],), 'p0.p1.p2.p3.p4.p5.p6.p7.p8.p9.p10.p11.'), ('gap in segments', ([['filename*0', 'a2'], ['filename*1', 'b'], ['filename*3', 'd']],), 'a2b'), ('leading zero index', ([['filename*0', 'a'], ['filename*1', 'b2'], ['filename*01', 'X']],), 'ab2'), ('charset only on first segment', ([['filename*0*', "utf-8''caf%C3"], ['filename*1*', '%A9_2.txt']],), 'café_2.txt'), ('quote inside plain first segment', ([['filename*0', "it's"], ['filename*1', 'fine2']],), "it'sfine2"), ('single extended value wins', ([['filename', 'plain.txt'], ['filename*', "utf-8''na%C3%AFve2.txt"], ['filename*0', 'seg']],), 'naïve2.txt'), ('orphan continuation', ([['filename', 'plain2.txt'], ['filename*1', 'orphan']],), 'plain2.txt'), ('upper-case names', ([['FileName*0', 'up'], ['FILENAME*1', 'per2']],), 'upper2')], 3: [('twelve segments', ([['filename*0', 'p0.'], ['filename*1', 'p1.'], ['filename*10', 'p10.'], ['filename*11', 'p11.'], ['filename*2', 'p2.'], ['filename*3', 'p3.'], ['filename*4', 'p4.'], ['filename*5', 'p5.'], ['filename*6', 'p6.'], ['filename*7', 'p7.'], ['filename*8', 'p8.'], ['filename*9', 'p9.'], ['filename', 'short3']],), 'p0.p1.p2.p3.p4.p5.p6.p7.p8.p9.p10.p11.'), ('gap in segments', ([['filename*0', 'a3'], ['filename*1', 'b'], ['filename*3', 'd']],), 'a3b'), ('leading zero index', ([['filename*0', 'a'], ['filename*1', 'b3'], ['filename*01', 'X']],), 'ab3'), ('charset only on first segment', ([['filename*0*', "utf-8''caf%C3"], ['filename*1*', '%A9_3.txt']],), 'café_3.txt'), ('quote inside plain first segment', ([['filename*0', "it's"], ['filename*1', 'fine3']],), "it'sfine3"), ('single extended value wins', ([['filename', 'plain.txt'], ['filename*', "utf-8''na%C3%AFve3.txt"], ['filename*0', 'seg']],), 'naïve3.txt'), ('orphan continuation', ([['filename', 'plain3.txt'], ['filename*1', 'orphan']],), 'plain3.txt'), ('upper-case names', ([['FileName*0', 'up'], ['FILENAME*1', 'per3']],), 'upper3')], 4: [('twelve segments', ([['filename*0', 'p0.'], ['filename*1', 'p1.'], ['filename*10', 'p10.'], ['filename*11', 'p11.'], ['filename*2', 'p2.'], ['filename*3', 'p3.'], ['filename*4', 'p4.'], ['filename*5', 'p5.'], ['filename*6', 'p6.'], ['filename*7', 'p7.'], ['filename*8', 'p8.'], ['filename*9', 'p9.'], ['filename', 'short4']],), 'p0.p1.p2.p3.p4.p5.p6.p7.p8.p9.p10.p11.'), ('gap in segments', ([['filename*0', 'a4'], ['filename*1', 'b'], ['filename*3', 'd']],), 'a4b'), ('leading zero index', ([['filename*0', 'a'], ['filename*1', 'b4'], ['filename*01', 'X']],), 'ab4'), ('charset only on first segment', ([['filename*0*', "utf-8''caf%C3"], ['filename*1*', '%A9_4.txt']],), 'café_4.txt'), ('quote inside plain first segment', ([['filename*0', "it's"], ['filename*1', 'fine4']],), "it'sfine4"), ('single extended value wins', ([['filename', 'plain.txt'], ['filename*', "utf-8''na%C3%AFve4.txt"], ['filename*0', 'seg']],), 'naïve4.txt'), ('orphan continuation', ([['filename', 'plain4.txt'], ['filename*1', 'orphan']],), 'plain4.txt'), ('upper-case names', ([['FileName*0', 'up'], ['FILENAME*1', 'per4']],), 'upper4')], 5: [('twelve segments', ([['filename*0', 'p0.'], ['filename*1', 'p1.'], ['filename*10', 'p10.'], ['filename*11', 'p11.'], ['filename*2', 'p2.'], ['filename*3', 'p3.'], ['filename*4', 'p4.'], ['filename*5', 'p5.'], ['filename*6', 'p6.'], ['filename*7', 'p7.'], ['filename*8', 'p8.'], ['filename*9', 'p9.'], ['filename', 'short5']],), 'p0.p1.p2.p3.p4.p5.p6.p7.p8.p9.p10.p11.'), ('gap in segments', ([['filename*0', 'a5'], ['filename*1', 'b'], ['filename*3', 'd']],), 'a5b'), ('leading zero index', ([['filename*0', 'a'], ['filename*1', 'b5'], ['filename*01', 'X']],), 'ab5'), ('charset only on first segment', ([['filename*0*', "utf-8''caf%C3"], ['filename*1*', '%A9_5.txt']],), 'café_5.txt'), ('quote inside plain first segment', ([['filename*0', "it's"], ['filename*1', 'fine5']],), "it'sfine5"), ('single extended value wins', ([['filename', 'plain.txt'], ['filename*', "utf-8''na%C3%AFve5.txt"], ['filename*0', 'seg']],), 'naïve5.txt'), ('orphan continuation', ([['filename', 'plain5.txt'], ['filename*1', 'orphan']],), 'plain5.txt'), ('upper-case names', ([['FileName*0', 'up'], ['FILENAME*1', 'per5']],), 'upper5')]}
for _label, _args, _expected in _CASES[N]:
    check(_label, solve(*_args), _expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
twelve segments	`p0.p1.p2.p3.p4.p5.p6.p7.p8.p9.p10.p11.`	`p0.p1.p2.p3.p4.p5.p6.p7.p8.p9.p10.p11.`	Passed
gap in segments	`a1b`	`a1b`	Passed
leading zero index	`ab1`	`ab1`	Passed
charset only on first segment	`café_1.txt`	`café_1.txt`	Passed
quote inside plain first segment	`it'sfine1`	`it'sfine1`	Passed
single extended value wins	`seg`	`naïve1.txt`	Failed
orphan continuation	`plain1.txt`	`plain1.txt`	Passed
upper-case names	`upper1`	`upper1`	Passed

SHA-256 / 1e4405238684057b8aaf4aabd52b697f6cb16bd2c8bf0983f9474d9254c9ef13

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json
from urllib.parse import unquote_to_bytes
N = 1
observations = []
def solve(params):
    single = None
    plain = None
    segs = {}
    for name, value in params:
        n = name.lower()
        if n == 'filename*':
            single = value
        elif n == 'filename':
            plain = value
        elif n.startswith('filename*'):
            rest = n[9:]
            enc = rest.endswith('*')
            num = rest[:-1] if enc else rest
            if num.isdigit() and (num == '0' or not num.startswith('0')):
                segs[int(num)] = (enc, value)
    def codec(cs):
        cs = (cs or 'us-ascii').lower()
        return cs if cs in ('utf-8', 'iso-8859-1', 'us-ascii') else 'utf-8'
    if single is not None:
        cs, _, rest = single.partition("'")
        _, _, value = rest.partition("'")
        return unquote_to_bytes(value).decode(codec(cs), 'replace')
    if 0 not in segs:
        return plain
    charset = 'us-ascii'
    chunks = []
    expect = 0
    for i in sorted(segs):
        if i != expect:
            break
        enc, v = segs[i]
        if enc and i == 0:
            charset, _, rest = v.partition("'")
            _, _, v = rest.partition("'")
        chunks.append(unquote_to_bytes(v) if enc else v.encode('latin-1', 'replace'))
        expect += 1
    return b''.join(chunks).decode(codec(charset), 'replace')
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('twelve segments', ([['filename*0', 'p0.'], ['filename*1', 'p1.'], ['filename*10', 'p10.'], ['filename*11', 'p11.'], ['filename*2', 'p2.'], ['filename*3', 'p3.'], ['filename*4', 'p4.'], ['filename*5', 'p5.'], ['filename*6', 'p6.'], ['filename*7', 'p7.'], ['filename*8', 'p8.'], ['filename*9', 'p9.'], ['filename', 'short1']],), 'p0.p1.p2.p3.p4.p5.p6.p7.p8.p9.p10.p11.'), ('gap in segments', ([['filename*0', 'a1'], ['filename*1', 'b'], ['filename*3', 'd']],), 'a1b'), ('leading zero index', ([['filename*0', 'a'], ['filename*1', 'b1'], ['filename*01', 'X']],), 'ab1'), ('charset only on first segment', ([['filename*0*', "utf-8''caf%C3"], ['filename*1*', '%A9_1.txt']],), 'café_1.txt'), ('quote inside plain first segment', ([['filename*0', "it's"], ['filename*1', 'fine1']],), "it'sfine1"), ('single extended value wins', ([['filename', 'plain.txt'], ['filename*', "utf-8''na%C3%AFve1.txt"], ['filename*0', 'seg']],), 'naïve1.txt'), ('orphan continuation', ([['filename', 'plain1.txt'], ['filename*1', 'orphan']],), 'plain1.txt'), ('upper-case names', ([['FileName*0', 'up'], ['FILENAME*1', 'per1']],), 'upper1')], 2: [('twelve segments', ([['filename*0', 'p0.'], ['filename*1', 'p1.'], ['filename*10', 'p10.'], ['filename*11', 'p11.'], ['filename*2', 'p2.'], ['filename*3', 'p3.'], ['filename*4', 'p4.'], ['filename*5', 'p5.'], ['filename*6', 'p6.'], ['filename*7', 'p7.'], ['filename*8', 'p8.'], ['filename*9', 'p9.'], ['filename', 'short2']],), 'p0.p1.p2.p3.p4.p5.p6.p7.p8.p9.p10.p11.'), ('gap in segments', ([['filename*0', 'a2'], ['filename*1', 'b'], ['filename*3', 'd']],), 'a2b'), ('leading zero index', ([['filename*0', 'a'], ['filename*1', 'b2'], ['filename*01', 'X']],), 'ab2'), ('charset only on first segment', ([['filename*0*', "utf-8''caf%C3"], ['filename*1*', '%A9_2.txt']],), 'café_2.txt'), ('quote inside plain first segment', ([['filename*0', "it's"], ['filename*1', 'fine2']],), "it'sfine2"), ('single extended value wins', ([['filename', 'plain.txt'], ['filename*', "utf-8''na%C3%AFve2.txt"], ['filename*0', 'seg']],), 'naïve2.txt'), ('orphan continuation', ([['filename', 'plain2.txt'], ['filename*1', 'orphan']],), 'plain2.txt'), ('upper-case names', ([['FileName*0', 'up'], ['FILENAME*1', 'per2']],), 'upper2')], 3: [('twelve segments', ([['filename*0', 'p0.'], ['filename*1', 'p1.'], ['filename*10', 'p10.'], ['filename*11', 'p11.'], ['filename*2', 'p2.'], ['filename*3', 'p3.'], ['filename*4', 'p4.'], ['filename*5', 'p5.'], ['filename*6', 'p6.'], ['filename*7', 'p7.'], ['filename*8', 'p8.'], ['filename*9', 'p9.'], ['filename', 'short3']],), 'p0.p1.p2.p3.p4.p5.p6.p7.p8.p9.p10.p11.'), ('gap in segments', ([['filename*0', 'a3'], ['filename*1', 'b'], ['filename*3', 'd']],), 'a3b'), ('leading zero index', ([['filename*0', 'a'], ['filename*1', 'b3'], ['filename*01', 'X']],), 'ab3'), ('charset only on first segment', ([['filename*0*', "utf-8''caf%C3"], ['filename*1*', '%A9_3.txt']],), 'café_3.txt'), ('quote inside plain first segment', ([['filename*0', "it's"], ['filename*1', 'fine3']],), "it'sfine3"), ('single extended value wins', ([['filename', 'plain.txt'], ['filename*', "utf-8''na%C3%AFve3.txt"], ['filename*0', 'seg']],), 'naïve3.txt'), ('orphan continuation', ([['filename', 'plain3.txt'], ['filename*1', 'orphan']],), 'plain3.txt'), ('upper-case names', ([['FileName*0', 'up'], ['FILENAME*1', 'per3']],), 'upper3')], 4: [('twelve segments', ([['filename*0', 'p0.'], ['filename*1', 'p1.'], ['filename*10', 'p10.'], ['filename*11', 'p11.'], ['filename*2', 'p2.'], ['filename*3', 'p3.'], ['filename*4', 'p4.'], ['filename*5', 'p5.'], ['filename*6', 'p6.'], ['filename*7', 'p7.'], ['filename*8', 'p8.'], ['filename*9', 'p9.'], ['filename', 'short4']],), 'p0.p1.p2.p3.p4.p5.p6.p7.p8.p9.p10.p11.'), ('gap in segments', ([['filename*0', 'a4'], ['filename*1', 'b'], ['filename*3', 'd']],), 'a4b'), ('leading zero index', ([['filename*0', 'a'], ['filename*1', 'b4'], ['filename*01', 'X']],), 'ab4'), ('charset only on first segment', ([['filename*0*', "utf-8''caf%C3"], ['filename*1*', '%A9_4.txt']],), 'café_4.txt'), ('quote inside plain first segment', ([['filename*0', "it's"], ['filename*1', 'fine4']],), "it'sfine4"), ('single extended value wins', ([['filename', 'plain.txt'], ['filename*', "utf-8''na%C3%AFve4.txt"], ['filename*0', 'seg']],), 'naïve4.txt'), ('orphan continuation', ([['filename', 'plain4.txt'], ['filename*1', 'orphan']],), 'plain4.txt'), ('upper-case names', ([['FileName*0', 'up'], ['FILENAME*1', 'per4']],), 'upper4')], 5: [('twelve segments', ([['filename*0', 'p0.'], ['filename*1', 'p1.'], ['filename*10', 'p10.'], ['filename*11', 'p11.'], ['filename*2', 'p2.'], ['filename*3', 'p3.'], ['filename*4', 'p4.'], ['filename*5', 'p5.'], ['filename*6', 'p6.'], ['filename*7', 'p7.'], ['filename*8', 'p8.'], ['filename*9', 'p9.'], ['filename', 'short5']],), 'p0.p1.p2.p3.p4.p5.p6.p7.p8.p9.p10.p11.'), ('gap in segments', ([['filename*0', 'a5'], ['filename*1', 'b'], ['filename*3', 'd']],), 'a5b'), ('leading zero index', ([['filename*0', 'a'], ['filename*1', 'b5'], ['filename*01', 'X']],), 'ab5'), ('charset only on first segment', ([['filename*0*', "utf-8''caf%C3"], ['filename*1*', '%A9_5.txt']],), 'café_5.txt'), ('quote inside plain first segment', ([['filename*0', "it's"], ['filename*1', 'fine5']],), "it'sfine5"), ('single extended value wins', ([['filename', 'plain.txt'], ['filename*', "utf-8''na%C3%AFve5.txt"], ['filename*0', 'seg']],), 'naïve5.txt'), ('orphan continuation', ([['filename', 'plain5.txt'], ['filename*1', 'orphan']],), 'plain5.txt'), ('upper-case names', ([['FileName*0', 'up'], ['FILENAME*1', 'per5']],), 'upper5')]}
for _label, _args, _expected in _CASES[N]:
    check(_label, solve(*_args), _expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
twelve segments	`p0.p1.p2.p3.p4.p5.p6.p7.p8.p9.p10.p11.`	`p0.p1.p2.p3.p4.p5.p6.p7.p8.p9.p10.p11.`	Passed
gap in segments	`a1b`	`a1b`	Passed
leading zero index	`ab1`	`ab1`	Passed
charset only on first segment	`café_1.txt`	`café_1.txt`	Passed
quote inside plain first segment	`it'sfine1`	`it'sfine1`	Passed
single extended value wins	`naïve1.txt`	`naïve1.txt`	Passed
orphan continuation	`plain1.txt`	`plain1.txt`	Passed
upper-case names	`upper1`	`upper1`	Passed

SHA-256 / e2f1abfbc08ff26d40836d82bcc876944ae73ecbd66fec0ad89acbfd267c34b0

Verification & scope

Stipulated offline model over pre-parsed MIME dictionaries; not a conforming MIME parser, generator or mail client. This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.

Observations recorded using Python 3.12.14 at 2026-09-29T14:49:18.494480+00:00.

Case digest / c1ab5600d141934ce001495a9bc7a43b3918041db640acd73075fc595da8eae7