fix(mattermost): use MIME types for media attachments

Bare strings like "image", "audio", "document" were appended to
media_types, but downstream run.py checks mtype.startswith("image/")
and mtype.startswith("audio/"), which never matched. This caused all
Mattermost file attachments to be silently dropped from vision/STT
processing. Use the actual MIME type from file_info instead.

tests/gateway/test_mattermost.py

@@ -572,3 +572,102 @@ class TestMattermostRequirements:
         monkeypatch.delenv("MATTERMOST_URL", raising=False)
         from gateway.platforms.mattermost import check_mattermost_requirements
         assert check_mattermost_requirements() is False
+
+
+# ---------------------------------------------------------------------------
+# Media type propagation (MIME types, not bare strings)
+# ---------------------------------------------------------------------------
+
+class TestMattermostMediaTypes:
+    """Verify that media_types contains actual MIME types (e.g. 'image/png')
+    rather than bare category strings ('image'), so downstream
+    ``mtype.startswith("image/")`` checks in run.py work correctly."""
+
+    def setup_method(self):
+        self.adapter = _make_adapter()
+        self.adapter._bot_user_id = "bot_user_id"
+        self.adapter.handle_message = AsyncMock()
+
+    def _make_event(self, file_ids):
+        post_data = {
+            "id": "post_media",
+            "user_id": "user_123",
+            "channel_id": "chan_456",
+            "message": "file attached",
+            "file_ids": file_ids,
+        }
+        return {
+            "event": "posted",
+            "data": {
+                "post": json.dumps(post_data),
+                "channel_type": "O",
+                "sender_name": "@alice",
+            },
+        }
+
+    @pytest.mark.asyncio
+    async def test_image_media_type_is_full_mime(self):
+        """An image attachment should produce 'image/png', not 'image'."""
+        file_info = {"name": "photo.png", "mime_type": "image/png"}
+        self.adapter._api_get = AsyncMock(return_value=file_info)
+
+        mock_resp = AsyncMock()
+        mock_resp.status = 200
+        mock_resp.read = AsyncMock(return_value=b"\x89PNG fake")
+        mock_resp.__aenter__ = AsyncMock(return_value=mock_resp)
+        mock_resp.__aexit__ = AsyncMock(return_value=False)
+        self.adapter._session = MagicMock()
+        self.adapter._session.get = MagicMock(return_value=mock_resp)
+
+        with patch("gateway.platforms.base.cache_image_from_bytes", return_value="/tmp/photo.png"):
+            await self.adapter._handle_ws_event(self._make_event(["file1"]))
+
+        msg = self.adapter.handle_message.call_args[0][0]
+        assert msg.media_types == ["image/png"]
+        assert msg.media_types[0].startswith("image/")
+
+    @pytest.mark.asyncio
+    async def test_audio_media_type_is_full_mime(self):
+        """An audio attachment should produce 'audio/ogg', not 'audio'."""
+        file_info = {"name": "voice.ogg", "mime_type": "audio/ogg"}
+        self.adapter._api_get = AsyncMock(return_value=file_info)
+
+        mock_resp = AsyncMock()
+        mock_resp.status = 200
+        mock_resp.read = AsyncMock(return_value=b"OGG fake")
+        mock_resp.__aenter__ = AsyncMock(return_value=mock_resp)
+        mock_resp.__aexit__ = AsyncMock(return_value=False)
+        self.adapter._session = MagicMock()
+        self.adapter._session.get = MagicMock(return_value=mock_resp)
+
+        with patch("gateway.platforms.base.cache_audio_from_bytes", return_value="/tmp/voice.ogg"), \
+             patch("gateway.platforms.base.cache_image_from_bytes"), \
+             patch("gateway.platforms.base.cache_document_from_bytes"):
+            await self.adapter._handle_ws_event(self._make_event(["file2"]))
+
+        msg = self.adapter.handle_message.call_args[0][0]
+        assert msg.media_types == ["audio/ogg"]
+        assert msg.media_types[0].startswith("audio/")
+
+    @pytest.mark.asyncio
+    async def test_document_media_type_is_full_mime(self):
+        """A document attachment should produce 'application/pdf', not 'document'."""
+        file_info = {"name": "report.pdf", "mime_type": "application/pdf"}
+        self.adapter._api_get = AsyncMock(return_value=file_info)
+
+        mock_resp = AsyncMock()
+        mock_resp.status = 200
+        mock_resp.read = AsyncMock(return_value=b"PDF fake")
+        mock_resp.__aenter__ = AsyncMock(return_value=mock_resp)
+        mock_resp.__aexit__ = AsyncMock(return_value=False)
+        self.adapter._session = MagicMock()
+        self.adapter._session.get = MagicMock(return_value=mock_resp)
+
+        with patch("gateway.platforms.base.cache_document_from_bytes", return_value="/tmp/report.pdf"), \
+             patch("gateway.platforms.base.cache_image_from_bytes"):
+            await self.adapter._handle_ws_event(self._make_event(["file3"]))
+
+        msg = self.adapter.handle_message.call_args[0][0]
+        assert msg.media_types == ["application/pdf"]
+        assert not msg.media_types[0].startswith("image/")
+        assert not msg.media_types[0].startswith("audio/")