FA-76401 / Email MIME structure / Open access

Resolve a part's effective content type and charset: type normalization · case 01

A mixed-case type such as Text/HTML is not recognised as text.

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

ROOT CAUSE

The type normalization decision evaluates `cand = head.strip()` where the contract requires `cand = head.strip().lower()`.

VERIFIED REPAIR

Use `cand = head.strip().lower()` for the type normalization decision and keep every other rule of the model unchanged.

Unsuccessful approach: Lower-casing without trimming leaves spaces that make a padded header invalid. The attempted `cand = head.lower()` still disagrees with a fixture.

Case contract

header is the raw Content-Type value or None. The type is the lower-cased text before ";" with surrounding spaces removed; it is valid only with a non-empty major and minor separated by "/" and no inner space. Missing or invalid types default to message/rfc822 inside multipart/digest and text/plain everywhere else (parameters of an invalid header are ignored). text/* types get the charset parameter (quotes removed, lower-cased) or us-ascii; other types have charset None. Result [type, charset].

Why this case matters

Default and fallback types decide whether a part is rendered as text, as a nested message or as an attachment.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(parent_type, header):
    default = 'message/rfc822' if parent_type == 'multipart/digest' else 'text/plain'
    mtype = default
    charset = None
    if header is not None:
        head, _, rest = header.partition(';')
        cand = head.strip()
        major, slash, minor = cand.partition('/')
        if slash and major and minor and ' ' not in cand:
            mtype = cand
            for piece in rest.split(';'):
                k, eq, v = piece.partition('=')
                if eq and k.strip().lower() == 'charset':
                    charset = v.strip().strip('"').lower()
    if mtype.startswith('text/'):
        charset = charset or 'us-ascii'
    else:
        charset = None
    return [mtype, charset]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('digest child without header', ('multipart/digest', None), ['message/rfc822', None]), ('mixed child without header', ('multipart/mixed', None), ['text/plain', 'us-ascii']), ('truncated type in digest', ('multipart/digest', 'text/'), ['message/rfc822', None]), ('missing major type', ('multipart/mixed', '/html; charset=utf-8'), ['text/plain', 'us-ascii']), ('quoted upper-case charset', ('multipart/mixed', 'Text/HTML; charset="UTF-8"'), ['text/html', 'utf-8']), ('charset on a binary type', ('multipart/mixed', 'image/png; charset=binary'), ['image/png', None]), ('padded header', ('multipart/alternative', ' text/plain ; charset=ISO-8859-1'), ['text/plain', 'iso-8859-1']), ('nested message parent', ('message/rfc822', None), ['text/plain', 'us-ascii'])], 2: [('digest child without header', ('multipart/digest', None), ['message/rfc822', None]), ('mixed child without header', ('multipart/mixed', None), ['text/plain', 'us-ascii']), ('truncated type in digest', ('multipart/digest', 'text/'), ['message/rfc822', None]), ('missing major type', ('multipart/mixed', '/html; charset=utf-8'), ['text/plain', 'us-ascii']), ('quoted upper-case charset', ('multipart/mixed', 'Text/HTML; charset="Windows-1252"'), ['text/html', 'windows-1252']), ('charset on a binary type', ('multipart/mixed', 'image/png; charset=binary'), ['image/png', None]), ('padded header', ('multipart/alternative', ' text/plain ; charset=ISO-8859-2'), ['text/plain', 'iso-8859-2']), ('nested message parent', ('message/rfc822', None), ['text/plain', 'us-ascii'])], 3: [('digest child without header', ('multipart/digest', None), ['message/rfc822', None]), ('mixed child without header', ('multipart/mixed', None), ['text/plain', 'us-ascii']), ('truncated type in digest', ('multipart/digest', 'text/'), ['message/rfc822', None]), ('missing major type', ('multipart/mixed', '/html; charset=utf-8'), ['text/plain', 'us-ascii']), ('quoted upper-case charset', ('multipart/mixed', 'Text/HTML; charset="KOI8-R"'), ['text/html', 'koi8-r']), ('charset on a binary type', ('multipart/mixed', 'image/png; charset=binary'), ['image/png', None]), ('padded header', ('multipart/alternative', ' text/plain ; charset=ISO-8859-3'), ['text/plain', 'iso-8859-3']), ('nested message parent', ('message/rfc822', None), ['text/plain', 'us-ascii'])], 4: [('digest child without header', ('multipart/digest', None), ['message/rfc822', None]), ('mixed child without header', ('multipart/mixed', None), ['text/plain', 'us-ascii']), ('truncated type in digest', ('multipart/digest', 'text/'), ['message/rfc822', None]), ('missing major type', ('multipart/mixed', '/html; charset=utf-8'), ['text/plain', 'us-ascii']), ('quoted upper-case charset', ('multipart/mixed', 'Text/HTML; charset="Shift_JIS"'), ['text/html', 'shift_jis']), ('charset on a binary type', ('multipart/mixed', 'image/png; charset=binary'), ['image/png', None]), ('padded header', ('multipart/alternative', ' text/plain ; charset=ISO-8859-4'), ['text/plain', 'iso-8859-4']), ('nested message parent', ('message/rfc822', None), ['text/plain', 'us-ascii'])], 5: [('digest child without header', ('multipart/digest', None), ['message/rfc822', None]), ('mixed child without header', ('multipart/mixed', None), ['text/plain', 'us-ascii']), ('truncated type in digest', ('multipart/digest', 'text/'), ['message/rfc822', None]), ('missing major type', ('multipart/mixed', '/html; charset=utf-8'), ['text/plain', 'us-ascii']), ('quoted upper-case charset', ('multipart/mixed', 'Text/HTML; charset="EUC-KR"'), ['text/html', 'euc-kr']), ('charset on a binary type', ('multipart/mixed', 'image/png; charset=binary'), ['image/png', None]), ('padded header', ('multipart/alternative', ' text/plain ; charset=ISO-8859-5'), ['text/plain', 'iso-8859-5']), ('nested message parent', ('message/rfc822', None), ['text/plain', 'us-ascii'])]}
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
digest child without header	`['message/rfc822', None]`	`['message/rfc822', None]`	Passed
mixed child without header	`['text/plain', 'us-ascii']`	`['text/plain', 'us-ascii']`	Passed
truncated type in digest	`['message/rfc822', None]`	`['message/rfc822', None]`	Passed
missing major type	`['text/plain', 'us-ascii']`	`['text/plain', 'us-ascii']`	Passed
quoted upper-case charset	`['Text/HTML', None]`	`['text/html', 'utf-8']`	Failed
charset on a binary type	`['image/png', None]`	`['image/png', None]`	Passed
padded header	`['text/plain', 'iso-8859-1']`	`['text/plain', 'iso-8859-1']`	Passed
nested message parent	`['text/plain', 'us-ascii']`	`['text/plain', 'us-ascii']`	Passed

SHA-256 / 1e9eff8e8852395593b78cd0336d0373f84d1138f736bcfd1a1812e14122bdee

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(parent_type, header):
    default = 'message/rfc822' if parent_type == 'multipart/digest' else 'text/plain'
    mtype = default
    charset = None
    if header is not None:
        head, _, rest = header.partition(';')
        cand = head.lower()
        major, slash, minor = cand.partition('/')
        if slash and major and minor and ' ' not in cand:
            mtype = cand
            for piece in rest.split(';'):
                k, eq, v = piece.partition('=')
                if eq and k.strip().lower() == 'charset':
                    charset = v.strip().strip('"').lower()
    if mtype.startswith('text/'):
        charset = charset or 'us-ascii'
    else:
        charset = None
    return [mtype, charset]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('digest child without header', ('multipart/digest', None), ['message/rfc822', None]), ('mixed child without header', ('multipart/mixed', None), ['text/plain', 'us-ascii']), ('truncated type in digest', ('multipart/digest', 'text/'), ['message/rfc822', None]), ('missing major type', ('multipart/mixed', '/html; charset=utf-8'), ['text/plain', 'us-ascii']), ('quoted upper-case charset', ('multipart/mixed', 'Text/HTML; charset="UTF-8"'), ['text/html', 'utf-8']), ('charset on a binary type', ('multipart/mixed', 'image/png; charset=binary'), ['image/png', None]), ('padded header', ('multipart/alternative', ' text/plain ; charset=ISO-8859-1'), ['text/plain', 'iso-8859-1']), ('nested message parent', ('message/rfc822', None), ['text/plain', 'us-ascii'])], 2: [('digest child without header', ('multipart/digest', None), ['message/rfc822', None]), ('mixed child without header', ('multipart/mixed', None), ['text/plain', 'us-ascii']), ('truncated type in digest', ('multipart/digest', 'text/'), ['message/rfc822', None]), ('missing major type', ('multipart/mixed', '/html; charset=utf-8'), ['text/plain', 'us-ascii']), ('quoted upper-case charset', ('multipart/mixed', 'Text/HTML; charset="Windows-1252"'), ['text/html', 'windows-1252']), ('charset on a binary type', ('multipart/mixed', 'image/png; charset=binary'), ['image/png', None]), ('padded header', ('multipart/alternative', ' text/plain ; charset=ISO-8859-2'), ['text/plain', 'iso-8859-2']), ('nested message parent', ('message/rfc822', None), ['text/plain', 'us-ascii'])], 3: [('digest child without header', ('multipart/digest', None), ['message/rfc822', None]), ('mixed child without header', ('multipart/mixed', None), ['text/plain', 'us-ascii']), ('truncated type in digest', ('multipart/digest', 'text/'), ['message/rfc822', None]), ('missing major type', ('multipart/mixed', '/html; charset=utf-8'), ['text/plain', 'us-ascii']), ('quoted upper-case charset', ('multipart/mixed', 'Text/HTML; charset="KOI8-R"'), ['text/html', 'koi8-r']), ('charset on a binary type', ('multipart/mixed', 'image/png; charset=binary'), ['image/png', None]), ('padded header', ('multipart/alternative', ' text/plain ; charset=ISO-8859-3'), ['text/plain', 'iso-8859-3']), ('nested message parent', ('message/rfc822', None), ['text/plain', 'us-ascii'])], 4: [('digest child without header', ('multipart/digest', None), ['message/rfc822', None]), ('mixed child without header', ('multipart/mixed', None), ['text/plain', 'us-ascii']), ('truncated type in digest', ('multipart/digest', 'text/'), ['message/rfc822', None]), ('missing major type', ('multipart/mixed', '/html; charset=utf-8'), ['text/plain', 'us-ascii']), ('quoted upper-case charset', ('multipart/mixed', 'Text/HTML; charset="Shift_JIS"'), ['text/html', 'shift_jis']), ('charset on a binary type', ('multipart/mixed', 'image/png; charset=binary'), ['image/png', None]), ('padded header', ('multipart/alternative', ' text/plain ; charset=ISO-8859-4'), ['text/plain', 'iso-8859-4']), ('nested message parent', ('message/rfc822', None), ['text/plain', 'us-ascii'])], 5: [('digest child without header', ('multipart/digest', None), ['message/rfc822', None]), ('mixed child without header', ('multipart/mixed', None), ['text/plain', 'us-ascii']), ('truncated type in digest', ('multipart/digest', 'text/'), ['message/rfc822', None]), ('missing major type', ('multipart/mixed', '/html; charset=utf-8'), ['text/plain', 'us-ascii']), ('quoted upper-case charset', ('multipart/mixed', 'Text/HTML; charset="EUC-KR"'), ['text/html', 'euc-kr']), ('charset on a binary type', ('multipart/mixed', 'image/png; charset=binary'), ['image/png', None]), ('padded header', ('multipart/alternative', ' text/plain ; charset=ISO-8859-5'), ['text/plain', 'iso-8859-5']), ('nested message parent', ('message/rfc822', None), ['text/plain', 'us-ascii'])]}
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
digest child without header	`['message/rfc822', None]`	`['message/rfc822', None]`	Passed
mixed child without header	`['text/plain', 'us-ascii']`	`['text/plain', 'us-ascii']`	Passed
truncated type in digest	`['message/rfc822', None]`	`['message/rfc822', None]`	Passed
missing major type	`['text/plain', 'us-ascii']`	`['text/plain', 'us-ascii']`	Passed
quoted upper-case charset	`['text/html', 'utf-8']`	`['text/html', 'utf-8']`	Passed
charset on a binary type	`['image/png', None]`	`['image/png', None]`	Passed
padded header	`['text/plain', 'us-ascii']`	`['text/plain', 'iso-8859-1']`	Failed
nested message parent	`['text/plain', 'us-ascii']`	`['text/plain', 'us-ascii']`	Passed

SHA-256 / 69db8c4119234d5fb81bd419596b7970cdefe40e97ab48af00a287e7387eb21a

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(parent_type, header):
    default = 'message/rfc822' if parent_type == 'multipart/digest' else 'text/plain'
    mtype = default
    charset = None
    if header is not None:
        head, _, rest = header.partition(';')
        cand = head.strip().lower()
        major, slash, minor = cand.partition('/')
        if slash and major and minor and ' ' not in cand:
            mtype = cand
            for piece in rest.split(';'):
                k, eq, v = piece.partition('=')
                if eq and k.strip().lower() == 'charset':
                    charset = v.strip().strip('"').lower()
    if mtype.startswith('text/'):
        charset = charset or 'us-ascii'
    else:
        charset = None
    return [mtype, charset]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('digest child without header', ('multipart/digest', None), ['message/rfc822', None]), ('mixed child without header', ('multipart/mixed', None), ['text/plain', 'us-ascii']), ('truncated type in digest', ('multipart/digest', 'text/'), ['message/rfc822', None]), ('missing major type', ('multipart/mixed', '/html; charset=utf-8'), ['text/plain', 'us-ascii']), ('quoted upper-case charset', ('multipart/mixed', 'Text/HTML; charset="UTF-8"'), ['text/html', 'utf-8']), ('charset on a binary type', ('multipart/mixed', 'image/png; charset=binary'), ['image/png', None]), ('padded header', ('multipart/alternative', ' text/plain ; charset=ISO-8859-1'), ['text/plain', 'iso-8859-1']), ('nested message parent', ('message/rfc822', None), ['text/plain', 'us-ascii'])], 2: [('digest child without header', ('multipart/digest', None), ['message/rfc822', None]), ('mixed child without header', ('multipart/mixed', None), ['text/plain', 'us-ascii']), ('truncated type in digest', ('multipart/digest', 'text/'), ['message/rfc822', None]), ('missing major type', ('multipart/mixed', '/html; charset=utf-8'), ['text/plain', 'us-ascii']), ('quoted upper-case charset', ('multipart/mixed', 'Text/HTML; charset="Windows-1252"'), ['text/html', 'windows-1252']), ('charset on a binary type', ('multipart/mixed', 'image/png; charset=binary'), ['image/png', None]), ('padded header', ('multipart/alternative', ' text/plain ; charset=ISO-8859-2'), ['text/plain', 'iso-8859-2']), ('nested message parent', ('message/rfc822', None), ['text/plain', 'us-ascii'])], 3: [('digest child without header', ('multipart/digest', None), ['message/rfc822', None]), ('mixed child without header', ('multipart/mixed', None), ['text/plain', 'us-ascii']), ('truncated type in digest', ('multipart/digest', 'text/'), ['message/rfc822', None]), ('missing major type', ('multipart/mixed', '/html; charset=utf-8'), ['text/plain', 'us-ascii']), ('quoted upper-case charset', ('multipart/mixed', 'Text/HTML; charset="KOI8-R"'), ['text/html', 'koi8-r']), ('charset on a binary type', ('multipart/mixed', 'image/png; charset=binary'), ['image/png', None]), ('padded header', ('multipart/alternative', ' text/plain ; charset=ISO-8859-3'), ['text/plain', 'iso-8859-3']), ('nested message parent', ('message/rfc822', None), ['text/plain', 'us-ascii'])], 4: [('digest child without header', ('multipart/digest', None), ['message/rfc822', None]), ('mixed child without header', ('multipart/mixed', None), ['text/plain', 'us-ascii']), ('truncated type in digest', ('multipart/digest', 'text/'), ['message/rfc822', None]), ('missing major type', ('multipart/mixed', '/html; charset=utf-8'), ['text/plain', 'us-ascii']), ('quoted upper-case charset', ('multipart/mixed', 'Text/HTML; charset="Shift_JIS"'), ['text/html', 'shift_jis']), ('charset on a binary type', ('multipart/mixed', 'image/png; charset=binary'), ['image/png', None]), ('padded header', ('multipart/alternative', ' text/plain ; charset=ISO-8859-4'), ['text/plain', 'iso-8859-4']), ('nested message parent', ('message/rfc822', None), ['text/plain', 'us-ascii'])], 5: [('digest child without header', ('multipart/digest', None), ['message/rfc822', None]), ('mixed child without header', ('multipart/mixed', None), ['text/plain', 'us-ascii']), ('truncated type in digest', ('multipart/digest', 'text/'), ['message/rfc822', None]), ('missing major type', ('multipart/mixed', '/html; charset=utf-8'), ['text/plain', 'us-ascii']), ('quoted upper-case charset', ('multipart/mixed', 'Text/HTML; charset="EUC-KR"'), ['text/html', 'euc-kr']), ('charset on a binary type', ('multipart/mixed', 'image/png; charset=binary'), ['image/png', None]), ('padded header', ('multipart/alternative', ' text/plain ; charset=ISO-8859-5'), ['text/plain', 'iso-8859-5']), ('nested message parent', ('message/rfc822', None), ['text/plain', 'us-ascii'])]}
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
digest child without header	`['message/rfc822', None]`	`['message/rfc822', None]`	Passed
mixed child without header	`['text/plain', 'us-ascii']`	`['text/plain', 'us-ascii']`	Passed
truncated type in digest	`['message/rfc822', None]`	`['message/rfc822', None]`	Passed
missing major type	`['text/plain', 'us-ascii']`	`['text/plain', 'us-ascii']`	Passed
quoted upper-case charset	`['text/html', 'utf-8']`	`['text/html', 'utf-8']`	Passed
charset on a binary type	`['image/png', None]`	`['image/png', None]`	Passed
padded header	`['text/plain', 'iso-8859-1']`	`['text/plain', 'iso-8859-1']`	Passed
nested message parent	`['text/plain', 'us-ascii']`	`['text/plain', 'us-ascii']`	Passed

SHA-256 / 937ad6c7c2b1dfe1d377beba22461086b4ba8ea288085303e37e9d34693b027e

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 2026-09-29T14:49:16.003356+00:00.

Case digest / dc637583e0ab5e35607f63fe3345c03fc5db13dd806252a877742ef37ffa5f1a