Arma 3 is a good game, I always try to make a mission myself but end up failure lmao. I remember Arma 3 has specific requirements for video files, using H.264 codec. You may need to use video editing software like Adobe Premiere, FoneDog Video Converter, or HandBrake to ensure proper encoding.
And what's the bitrate of your video? Don't know about Arma 3 maximum supported bitrate, 60FPS, or just 30. I think you may need to experiment with that.