fix(mattermost): set message type to DOCUMENT when post has file attachments

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.

gateway/platforms/mattermost.py

@@ -661,6 +661,8 @@ class MattermostAdapter(BasePlatformAdapter):
         msg_type = MessageType.TEXT
         if message_text.startswith("/"):
             msg_type = MessageType.COMMAND
+        elif file_ids:
+            msg_type = MessageType.DOCUMENT
 
         # Download file attachments immediately (URLs require auth headers
         # that downstream tools won't have).