Built in viewer video looping

Make sure the file extension you're trying to play is in its list. (Click Configure to edit the list.)

You'll also need a 64-bit codec (and possibly DirectShow splitter) for the format, if you are using 64-bit Opus. If those are missing (or not registered properly) then the Movie plugin will fail to show the file and Opus will fall back on the ActiveX plugin (which uses 32-bit Windows Media Player).

The Media Playback FAQ may help.