Store MIME type in database during upload

Detect actual MIME type from file content and store it instead of
generic "media" type. This enables proper media categorization
and filtering in the UI.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/services/cache_service.py

@@ -463,16 +463,19 @@ class CacheService:
                 tmp.write(content)
                 tmp_path = Path(tmp.name)
 
+            # Detect MIME type before moving file
+            mime_type = get_mime_type(tmp_path)
+
             # Store in cache
             cached, ipfs_cid = self.cache.put(tmp_path, node_type="upload", move=True)
             content_hash = cached.content_hash
 
-            # Save to database
+            # Save to database with detected MIME type
             await self.db.create_cache_item(content_hash, ipfs_cid)
             await self.db.save_item_metadata(
                 content_hash=content_hash,
                 actor_id=actor_id,
-                item_type="media",
+                item_type=mime_type,  # Store actual MIME type
                 filename=filename
             )