The latest firmware (2.02.32) for the WD TV Live broke the ability to play MP4 files. The forum users suggest either loading the older firmware (2.01.86) or removing the metadata from your MP4 files. The latter option is very quick with FFMPEG.
ffmpeg -i dance.mp4 -map_metadata -1 -map_chapters -1 -c:v copy -c:a copy dance2.mp4
Here is a link to the forum: