I came to report the same issue; it looks like your video file probably has a preview image added to it, which was the case with the ones I'm having trouble with as well. In my case the mp4's were created by Handbrake in the first place, except I then used a tagging tool (MetaZ) to add various tags and a preview image, since I'm going to be making the video availably digitally as well as burning DVDs for the handful of people have requested a physical copy. In my case I just ended up setting the...
So first of all, just wanted to say thanks for postfix admin, it made setting up...