I can at least tell you that MKV files are supported, most of the videos I've been playing on my N900 are using the MKV container. However, you can put stuff in an MKV that the N900 can't handle. I don't know exactly what the N900 can and can't deal with, but other recent threads here have been saying that you can't play HD quality video on it. (I'm not certain that is absolutely true, but I do know that I have trouble playing the few HD resolution videos I've tried.)

So if that is a 720p video you've got, you might need to convert it to a lower resolution in order to play it.