The mime type on that link is video/ogg, not video/x-theora. But you shouldn't have to add MIME types to the configuration manually, the plugin is just supposed to handle everything it supports automatically.
If it only works when you add mplayer.desktop to the configuration, the videos are likely playing directly in mplayer, not through the plugin. In any case, you should really try gecko-mediaplayer instead of this package, since it's actually maintained upstream. -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org