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Resolve a part's effective content type and charset: invalid type fallback · case 04

A malformed type such as "text/" or "/html" is used verbatim instead of falling back to the default.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

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.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
truncated type in digest	`["text/", "us-ascii"]`	`["message/rfc822", null]`	Failed

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