email.headerregistry fails to combine RFC 2231 continuations with mixed-case parameter names

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: The email.headerregistry documentation, under ParameterizedMIMEHeader.params, states: “Parameter names are always mapped to lower case” and “Parameter values are decoded if necessary according to RFC 2231.” RFC 2231 continuation decoding applies to the case-insensitive MIME parameter names.

Expected: params['filename'] == 'helloworld'

Actual: params['filename'] == 'world'

from email.headerregistry import HeaderRegistry

a, b = "hello", "world"
parts = [("filename*0*", "us-ascii''" + a), ("FILENAME*1*", b)]
value = "attachment; " + "; ".join(k + "=" + v for k, v in parts)
valid = all(s and s.isascii() and s.isalpha() for s in (a, b))
valid &= [k.lower() for k, _ in parts] == ["filename*0*", "filename*1*"]
if not valid:
    print("REFUTATION REJECTED: invalid RFC 2231 continuation input")
else:
    expected = a + b
    actual = HeaderRegistry()("Content-Disposition", value).params.get("filename")
    if actual != expected:
        print("REFUTATION CONFIRMED:", repr(value), "actual =", repr(actual),
              "expected =", repr(expected))
    else:
        print("REFUTATION REJECTED: actual matches documented expectation")

Output on Python 3.14.6 (Windows-11-10.0.26220-SP0), standard library email.headerregistry:

REFUTATION CONFIRMED: "attachment; filename*0*=us-ascii''hello; FILENAME*1*=world" actual = 'world' expected = 'helloworld'

This report was found and written by an automated property-testing tool I run (bugforge). The reproducer above was executed and its output is pasted unedited; no person reviewed the report before it was filed. The search script is in https://github.com/augusto-rehfeldt/bugforge-results/tree/main/email.headerregistry-20261003-195236-c1

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).