Lossless trim (copy) changes file format?

[miketunes]

I have a few mpeg4 clips that seem to change after I trim them.  My sources are a gopro2 and galaxy s3.  I set video and audio to copy, output format to avi muxer (auto).  I can trim and play in VLC fine.  But when i open with windows media player they will not play (or play with just audio).  The original files play fine in windows media player.  I also noticed the thumbnail disapears from windows (not a big deal, but it's probably indicating something changed.)  I also can't import the trimmed files into cyberlink powerdirector for further editing. I'm assuming it's something with the muxer, but I've tried different options without luck.  I've also tried avi and mp4 extensions.  Any ideas?

[miketunes]

OK, I fixed it.. so if anyone else has this issue changing to the MP4 Muxer, didn't like AVI muxer.