Ok, thanks! Here's the currently tested list of file formats that should work (as long as you have the codecs for them):
.3g2, .3gp, .3gp2, .3gpp, .amv, .avi, .divx, .dv, .evo, .f4v, .f4p, .f4a, .f4b, .flv, .gif, .hdmov, .m1v, .m2p, .m2t, .m2ts, .m4v, .mjpeg, .mkv, .mod, .mov, .mp2v, .mp4, .mp4v, .mpa, .mpeg, .mpg, .mpv4, .mts, .ogg, .ogm, .ogv, .rm, .rv, .tp, .tps, .trp, .ts, .tts, .vob, .wm, .wmv
If you change the drop-down to "All Files" in the Open File dialog, you can also select .webm and they should work. We'll add it to the list for the next beta.
If you haven't already, could you try installing a codec pack, like the
K-Lite Codec Pack to see if that helps with the video playback?