I beleive the consensus is, the avi convert from film viewer is hiy and miss at best, IMHO best move is, grab some screen recording software, OBS, Shadowplay, Action!, Bandicam etc, open your film in the viewer, hit record on your software, play in the viewer and go fullscreen in the viewer, basically re-record it.
You should then be able to save an mp4 from your software and go from there