Its a matroska video file format (format from russia i think)
If you use TCPMP Player on your PDA it also works in .mkv file format.
If you like to convert it into a avi (mp4,xvid - great pic quality) format use the PocketDivxEncoder
Download from here:
http://divx.ppccool.com/
(its multilingual)
some screenshots of this software:
http://www.pocketpc-blog.de/pocketdivxencoder.html
Its Freeware