The completion-document endpoint persisted whatever file it received: the
only checks were non-null, non-empty, and a 30 MB request limit. Its sibling
media endpoint has enforced a MIME allowlist, MIME-to-extension pairing, and
a magic-byte signature check since SH-116.
Validate before the file reaches storage, so a rejected upload leaves nothing
behind. An undetermined content type is accepted only alongside a .pdf name
and a %PDF- signature, because the browser leaves File.type empty when the OS
cannot classify the file and the completion-doc dialog already allows that.
