The .mp4 file likely uses H.264 encoding, which is also not supported right now. The only format that we found to play reliably is MPEG, which is what we use in our own .mp4 files.
We are currently working with Microsoft to improve the player plug-in for Windows. It will likely be rewritten, so it also supports XboxOne, but probably for 4.8 at the earliest as we are currently trying to finish up some other cool new things.