Fix default media_mime for media events

TODO.md

@@ -4,7 +4,6 @@
 - highlight messages with mention
 - add room members loading indicator
 
-- fix media_mime
 - fix lag when clicking accounts in the AccountBar with a very long room list
 - fix: on startup, if a room's last event is a membership change,
   it won't be visible in timeline no matter what the user config is

src/backend/nio_callbacks.py

@@ -149,7 +149,7 @@ class NioCallbacks:
             media_height     = info.get("h") or 0,
             media_duration   = info.get("duration") or 0,
             media_size       = info.get("size") or 0,
-            media_mime       = info.get("mimetype") or 0,
+            media_mime       = info.get("mimetype") or "",
             media_crypt_dict = media_crypt_dict,
 
             thumbnail_url =