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Reassemble RFC 2231 style filename parameter continuations: single extended precedence · case 05

Continuation segments override a complete filename* value.

Member previewVariant 5 · 3 implementations · 8 checks per implementation

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.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
single extended value wins	`"seg"`	`"na\u00efve5.txt"`	Failed

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