This extension is already present in WiiMC.
Here is the list of extension supported:
Video
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
Audio
aac, ac3, aif, aiff, amr, ape, apl, asf, flac, m4a, m4b, m4p, mka, mp2, mp3, mpc, ogg, ra, ram, rm, shn, wav, wma, wv
Pictures
bmp, jpeg, jpg, png, gif
Playlist
asx, m3u, pls, plx, ram, smil
