Use the program called Mediainfo in the software Links section to find the contents.http://mediainfo.sourceforge.net/en
The simple answer is a hardware player only has a limited set of codecs, and can only play a file using those codecs. So only AVI's play that contain a supported codec. The same with any package format.
It's best to start testing using a strict format, such a DVD.ISO created using DVD shrink.
Confused! I thought the whole point of the Mede8er was that it played anything.... .avi is such a basic/everyday format that you would think that codec would be included as a minimum. Is there a walk through on how to install mediainfo on the 500x2?