After upgading from REL1_27 to REL1_31 I realized that $wgMediaViewerUseThumbnailGuessing = true; does no longer work. The other setup especially for the 404 handler did not change. Either the smallest resolution is picked for up-scaling of the image resulting in a very pixeled view or alternatively the image is not being scaled at all resulting in a tiny thumb in the view. After changing the setting back to the default false everything works again. This happened for several wikis. On REL1_27 everything was cool.
