You can side load apps onto the Fire TV, for video files I would side load MX Player. When trying to play a video file Mezzmo (Trial) will send a message to the android OS and the app capable of playing the content will be launched. If there are multiple apps to handle the content then you will get a choice from a list of which app to use so if you install MX Player using side loading you should be prompted when playing a video through Mezzmo (Trial) allowing you to select MX Player.