Follow-up to the accept-any-file-type change. The observe-unmentioned and
replied-media paths relied on cache_media_bytes() returning None for
unsupported document types to emit an 'unsupported, not cached' note. Now
that any file type is always cached, those docs are cached and surfaced with
a path-pointing note — consistent with the main document path. The
remaining cached-is-None branch is image-validation-failure only; its note
is reworded accordingly. Updates the group-gating test to the new contract.
b5bd66eac9