The Mattermost adapter downloads file attachments correctly but
never updates msg_type from TEXT to DOCUMENT. This means the
document enrichment block in gateway/run.py (which requires
MessageType.DOCUMENT) never executes — text files are not
inlined, and the agent is never notified about attached files.
The user sends a file, the adapter downloads it to the local
cache, but the agent sees an empty message and responds with
'I didn't receive any file'.
Set msg_type to DOCUMENT when file_ids is non-empty, matching
the behavior of the Telegram and Discord adapters.
3282b7066c