Just found this. Not sure what it all means but maybe someone will.
Supported file types
The following table lists the file types (formats) that are supported by Windows Media Player.
If you cannot use a file that is included in this table, the file might have been encoded with a codec that is not supported by Windows Media Player.
File type (format)
File name extension
AU (UNIX) audio files
.au and .snd
Audio Interchange File Format (AIFF) audio files
.aif, .aifc, and .aiff
CD audio disc
.cda
DVD-Video disc
.vob
JPEG picture file
.jpg
Macromedia Flash animation file
.swf
Microsoft Recorded TV Show
.dvr-ms
MP3 audio files
.mp3 and .m3u
MPEG video files
.mpeg, .mpg, .m1v, .mp2, .mpa, .mpe, .mp2v, and .mpv2
Musical Instrument Digital Interface (MIDI) audio files
.mid, .midi, and .rmi
Windows audio and video files
.avi and .wav
Windows Media audio and video files
.asf, .asx, .wax, .wm, .wma, .wmd, .wmp, .wmv, .wmx, .wpl, and .wvx
Windows Media Player skin file
.wmz