FA-76566 / Email MIME structure / Open access

Choose the MIME skeleton for an outgoing message: mixed only for attachments · case 01

Messages with inline images but no attachments get an unnecessary multipart/mixed wrapper.

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ROOT CAUSE

The mixed only for attachments decision evaluates `if atts or inline_images:` where the contract requires `if atts:`.

THE FAILURE

The mixed only for attachments decision evaluates `if atts or inline_images:` where the contract requires `if atts:`.

Unsuccessful approach: Wrapping only messages with several images is still wrong for those messages. The attempted `if atts or inline_images > 1:` still disagrees with a fixture.

Case contract

The body is multipart/alternative(text, html) when both versions exist, the html alone when only html exists, else text. Inline images go into multipart/related with the html root first; without html they become attachments. When there are attachments the result is multipart/mixed(body, attachments...). Output is a compact string such as "mixed(alternative(text,related(html,img)),att)".

Why this case matters

Composing the wrong skeleton makes clients show images as attachments, hide the plain-text version or drop parts.

1 / The failure

Exit 1

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

N = 1
observations = []
def solve(has_text, has_html, inline_images, attachments):
    atts = attachments
    html = None
    if has_html:
        html = 'html' if inline_images == 0 else 'related(' + ','.join(['html'] + ['img'] * inline_images) + ')'
    else:
        atts += inline_images
    if has_text and html:
        body = 'alternative(text,' + html + ')'
    elif html:
        body = html
    else:
        body = 'text'
    if atts or inline_images:
        return 'mixed(' + ','.join([body] + ['att'] * atts) + ')'
    return body
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('images without html', (False, False, 2, 0), 'mixed(text,att,att)'), ('text and html', (True, True, 0, 0), 'alternative(text,html)'), ('text html and images', (True, True, 2, 0), 'alternative(text,related(html,img,img))'), ('html images and attachment', (False, True, 1, 1), 'mixed(related(html,img),att)'), ('text and attachments', (True, False, 0, 1), 'mixed(text,att)'), ('plain text only', (True, False, 0, 0), 'text'), ('everything', (True, True, 2, 1), 'mixed(alternative(text,related(html,img,img)),att)')], 2: [('images without html', (False, False, 3, 0), 'mixed(text,att,att,att)'), ('text and html', (True, True, 0, 0), 'alternative(text,html)'), ('text html and images', (True, True, 3, 0), 'alternative(text,related(html,img,img,img))'), ('html images and attachment', (False, True, 2, 1), 'mixed(related(html,img,img),att)'), ('text and attachments', (True, False, 0, 2), 'mixed(text,att,att)'), ('plain text only', (True, False, 0, 0), 'text'), ('everything', (True, True, 2, 2), 'mixed(alternative(text,related(html,img,img)),att,att)')], 3: [('images without html', (False, False, 4, 0), 'mixed(text,att,att,att,att)'), ('text and html', (True, True, 0, 0), 'alternative(text,html)'), ('text html and images', (True, True, 4, 0), 'alternative(text,related(html,img,img,img,img))'), ('html images and attachment', (False, True, 3, 1), 'mixed(related(html,img,img,img),att)'), ('text and attachments', (True, False, 0, 3), 'mixed(text,att,att,att)'), ('plain text only', (True, False, 0, 0), 'text'), ('everything', (True, True, 2, 3), 'mixed(alternative(text,related(html,img,img)),att,att,att)')], 4: [('images without html', (False, False, 5, 0), 'mixed(text,att,att,att,att,att)'), ('text and html', (True, True, 0, 0), 'alternative(text,html)'), ('text html and images', (True, True, 5, 0), 'alternative(text,related(html,img,img,img,img,img))'), ('html images and attachment', (False, True, 4, 1), 'mixed(related(html,img,img,img,img),att)'), ('text and attachments', (True, False, 0, 4), 'mixed(text,att,att,att,att)'), ('plain text only', (True, False, 0, 0), 'text'), ('everything', (True, True, 2, 4), 'mixed(alternative(text,related(html,img,img)),att,att,att,att)')], 5: [('images without html', (False, False, 6, 0), 'mixed(text,att,att,att,att,att,att)'), ('text and html', (True, True, 0, 0), 'alternative(text,html)'), ('text html and images', (True, True, 6, 0), 'alternative(text,related(html,img,img,img,img,img,img))'), ('html images and attachment', (False, True, 5, 1), 'mixed(related(html,img,img,img,img,img),att)'), ('text and attachments', (True, False, 0, 5), 'mixed(text,att,att,att,att,att)'), ('plain text only', (True, False, 0, 0), 'text'), ('everything', (True, True, 2, 5), 'mixed(alternative(text,related(html,img,img)),att,att,att,att,att)')]}
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
images without html	`mixed(text,att,att)`	`mixed(text,att,att)`	Passed
text and html	`alternative(text,html)`	`alternative(text,html)`	Passed
text html and images	`mixed(alternative(text,related(html,img,img)))`	`alternative(text,related(html,img,img))`	Failed
html images and attachment	`mixed(related(html,img),att)`	`mixed(related(html,img),att)`	Passed
text and attachments	`mixed(text,att)`	`mixed(text,att)`	Passed
plain text only	`text`	`text`	Passed
everything	`mixed(alternative(text,related(html,img,img)),att)`	`mixed(alternative(text,related(html,img,img)),att)`	Passed

SHA-256 / 9de920167b34cf42103da4faeb94b9bc10432c8d56692fbd0310d0bd0fbfe56d

2 / The unsuccessful fix

Exit 1

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

N = 1
observations = []
def solve(has_text, has_html, inline_images, attachments):
    atts = attachments
    html = None
    if has_html:
        html = 'html' if inline_images == 0 else 'related(' + ','.join(['html'] + ['img'] * inline_images) + ')'
    else:
        atts += inline_images
    if has_text and html:
        body = 'alternative(text,' + html + ')'
    elif html:
        body = html
    else:
        body = 'text'
    if atts or inline_images > 1:
        return 'mixed(' + ','.join([body] + ['att'] * atts) + ')'
    return body
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('images without html', (False, False, 2, 0), 'mixed(text,att,att)'), ('text and html', (True, True, 0, 0), 'alternative(text,html)'), ('text html and images', (True, True, 2, 0), 'alternative(text,related(html,img,img))'), ('html images and attachment', (False, True, 1, 1), 'mixed(related(html,img),att)'), ('text and attachments', (True, False, 0, 1), 'mixed(text,att)'), ('plain text only', (True, False, 0, 0), 'text'), ('everything', (True, True, 2, 1), 'mixed(alternative(text,related(html,img,img)),att)')], 2: [('images without html', (False, False, 3, 0), 'mixed(text,att,att,att)'), ('text and html', (True, True, 0, 0), 'alternative(text,html)'), ('text html and images', (True, True, 3, 0), 'alternative(text,related(html,img,img,img))'), ('html images and attachment', (False, True, 2, 1), 'mixed(related(html,img,img),att)'), ('text and attachments', (True, False, 0, 2), 'mixed(text,att,att)'), ('plain text only', (True, False, 0, 0), 'text'), ('everything', (True, True, 2, 2), 'mixed(alternative(text,related(html,img,img)),att,att)')], 3: [('images without html', (False, False, 4, 0), 'mixed(text,att,att,att,att)'), ('text and html', (True, True, 0, 0), 'alternative(text,html)'), ('text html and images', (True, True, 4, 0), 'alternative(text,related(html,img,img,img,img))'), ('html images and attachment', (False, True, 3, 1), 'mixed(related(html,img,img,img),att)'), ('text and attachments', (True, False, 0, 3), 'mixed(text,att,att,att)'), ('plain text only', (True, False, 0, 0), 'text'), ('everything', (True, True, 2, 3), 'mixed(alternative(text,related(html,img,img)),att,att,att)')], 4: [('images without html', (False, False, 5, 0), 'mixed(text,att,att,att,att,att)'), ('text and html', (True, True, 0, 0), 'alternative(text,html)'), ('text html and images', (True, True, 5, 0), 'alternative(text,related(html,img,img,img,img,img))'), ('html images and attachment', (False, True, 4, 1), 'mixed(related(html,img,img,img,img),att)'), ('text and attachments', (True, False, 0, 4), 'mixed(text,att,att,att,att)'), ('plain text only', (True, False, 0, 0), 'text'), ('everything', (True, True, 2, 4), 'mixed(alternative(text,related(html,img,img)),att,att,att,att)')], 5: [('images without html', (False, False, 6, 0), 'mixed(text,att,att,att,att,att,att)'), ('text and html', (True, True, 0, 0), 'alternative(text,html)'), ('text html and images', (True, True, 6, 0), 'alternative(text,related(html,img,img,img,img,img,img))'), ('html images and attachment', (False, True, 5, 1), 'mixed(related(html,img,img,img,img,img),att)'), ('text and attachments', (True, False, 0, 5), 'mixed(text,att,att,att,att,att)'), ('plain text only', (True, False, 0, 0), 'text'), ('everything', (True, True, 2, 5), 'mixed(alternative(text,related(html,img,img)),att,att,att,att,att)')]}
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
images without html	`mixed(text,att,att)`	`mixed(text,att,att)`	Passed
text and html	`alternative(text,html)`	`alternative(text,html)`	Passed
text html and images	`mixed(alternative(text,related(html,img,img)))`	`alternative(text,related(html,img,img))`	Failed
html images and attachment	`mixed(related(html,img),att)`	`mixed(related(html,img),att)`	Passed
text and attachments	`mixed(text,att)`	`mixed(text,att)`	Passed
plain text only	`text`	`text`	Passed
everything	`mixed(alternative(text,related(html,img,img)),att)`	`mixed(alternative(text,related(html,img,img)),att)`	Passed

SHA-256 / 1f4d0ffe93abfee433bc587d20123445cdbb6396e25c660078a4e9482fdde57e

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This mechanism has 7 recorded checks per implementation. The open-access tier publishes the failure and the unsuccessful fix; the repaired source that passes every check, and the observations that prove it, are available to members.

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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 2026-09-29T14:49:17.457935+00:00.

Case digest / 96c2bc7b52cc491a3f8b05a952d3419d2baaaacc5bf3cee186c220d6aae4b864