DVD playback, with DVD menu support (newer Wiis have incompatible drives)
Files with AVC/H.264 video streams (typically .mp4, .mkv or .flv's from YouTube or other sites) with resolutions up to 720x480p usually play correctly.
Files with MPEG-4 Simple video streams (typically XVID/DIVX .avi's) with resolutions up to 656x352 play correctly.
Files with MPEG-2 PS video streams (typically DVD rips and TV-DVR recorded .mpg's) with resolutions up to 720x480 play correctly.
Files with MPEG-2 TS video streams (typically HDTV recorded .ts files) with resolutions up to 1280x720 play correctly.
AAC, AC3, FLAC, MP3, OGG Vorbis, WavPack, and WAV PCM Audio Streams all play correctly.
Supported File Extensions
3gp, asf, avi, bik, divx, dpg, dv, dvr-ms, evo, f4v, flc, fli, flv, iso, m1v, m2t, m2ts, m2v, m4v, mkv, mov, mp4, mp4v, mpe, mpeg, mpg, mqv, nsv, nuv, ogm, qt, rmvb, swf, trp, ts, vdr, vfw, viv, vob, vro, wmv, y4m (maximum allowed resolution: 1280x720)
aac, ac3, aif, aiff, amr, ape, apl, asf, flac, m4a, m4b, m4p, mka, mp2, mp3, mpc, ogg, ra, ram, rm, shn, wav, wma, wv
bmp, jpeg, jpg, png, gif
asx, m3u, pls, plx, ram, smil
Information from WiiBrew