Specifically, it'd be nice if flv files were recognized since I often download things from youtube to play for my daughter. But in general, mplayer will work with a whole bunch of media types which the tab completion doesn't handle. Maybe better to complete on all files? Or, make a blacklist of things which it's known *not* to play? :)
Even though it's pretty hard to keep up with everything media players support, I think there's still value in the current approach. flv and some other improvements added in 20060301-11, for both mplayer and xine based players.