FA-76818 / Email MIME structure / Member archive

Strip blocked attachments under a content policy: size boundary · case 03

Attachments exactly at the size limit are stripped.

Member previewVariant 3 · 3 implementations · 7 checks per implementation

Case contract

Leaves {type, filename, disp, size}. A leaf is blocked when its last filename extension (trimmed, lower-cased) is in blocked_ext, its type is application/x-msdownload, or its size exceeds max_size. A non-attachment leaf without a filename (a body) is never blocked. Blocked leaves become "strip", others "keep"; a blocked leaf anywhere under multipart/signed cannot be stripped, and then the whole message is "reject". Result: the per-leaf action list or "reject".

Why this case matters

Mail gateways strip dangerous attachments but must not silently break signed messages.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
size boundaries	`["keep", "strip", "strip"]`	`["keep", "keep", "strip"]`	Failed

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