Log responder we are using. (#6139)

This prevents us logging "Responding to media request with responder %s".

changelog.d/6139.misc

+Log responder when responding to media request.

synapse/rest/media/v1/_base.py

@@ -195,7 +195,7 @@ def respond_with_responder(request, responder, media_type, file_size, upload_nam
         respond_404(request)
         return
 
-    logger.debug("Responding to media request with responder %s")
+    logger.debug("Responding to media request with responder %s", responder)
     add_file_headers(request, media_type, file_size, upload_name)
     try:
         with responder: