Codex found a SILENT edge in the payload-signature fix: a live browser File
JSON-serializes to {} when the draft is POSTed to the server, so a text+attachment
send's suppression signature (computed from the rich File) never matched the
restored server draft (files:[{}]) — suppression cleared and the just-sent tail
could repopulate for the attachment case. Fix: _composerDraftFilesForPersist()
canonicalizes files to a stable serializable shape (name/path/size/type/lastModified)
used at BOTH _saveComposerDraft/_saveComposerDraftNow (persist) AND
_composerDraftPayloadSignature (sign), so the sent signature matches the
round-tripped server draft. Regression test self-demonstrates lossy-mismatch vs
canon-match.
Co-authored-by: ruizanthony <ruizanthony@users.noreply.github.com>
