supports HTML5 video
The TL;DR is that for especially large captures, the form was taking them to the submit page before the thumbnail could find its API target and plant itself in the CDN. So now I've finaggled it so that it stays loading as long as it has to.
I tried it with one particular file that gave me trouble before - and now it works. Maybe I've solved the MD thumbnail crisis.
BONOOOOOOO, why isn't the top list indexed properly yet!?