Alright, it looks like APNG files act like GIF files in the sense that they won’t play (or for GIFs, upload) unless the file is under 1MB. The Frigid swaying APNG was already 750kB, and my other practical attempts were over 3MB (such as other animated posts that were dumbed down to GIFs). I’ll still be researching this though, and updating my findings onto Tumblr!