From what I have read on here the current iteration of Media Share (the function of the DirecTV receivers that allows you to play music/movie files) supports very few codecs. Some people have used programs (like TVersity and PlayOn I think) to transcode the files in realtime in order to get it to work, but have varying results. My gut is telling me you probably won't have much luck with this feature at this time. However, I believe a few people have said that DirecTV will eventually upgrade Media Share to gain more functionality, but there isn't a real timeline so I'm not sure you would want to rely on it.
Your best bet is probably to get something like a ROKU, or AppleTV and use them instead. They are relatively cheap and give you more options like Netflix, Hulu, etc.