when you save in the materials room you get a prompt. If you do it right for a collection  you get an mc6 file extension, otherwise you save one material only as an mt5 extension.  I discard the thumbnail for the material and render one for the mat pose. They are just 91 x 91 png files .

Mat pose edit may be of interest http://koti.mbnet.fi/~ilaripih/MPEHome/

http://poserutils.tripod.com/poserutils.html     mat writer python