I have a 2011 D 6420, with the PLEX app installed. I have successfully added movies to my server using MakeMKV. (I am new to the server world, and ripping.) They play perfectly. (dvd's, not Blu-ray...) I have done Avengers, Star Trek, Skyfall, Dark Knight Rises, Hotel Transylvania. My personal music library, (lossless WMA), photo library, etc...
It's a little difficult to navigate, sometimes, with the folder layout. (not sure how many movies you have...)
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Xbox LIVE Gamertag: KEVIN G 40