I don't use animations, but my understanding is this: You aren't (just) uploading a file. You are uploading a file, it is being "optimized"/converted for use on the vtt, and possibly multiple sizes of it are being generated (full size, half, thumbnail). From what I have seen pass the forums, this may take multiple hours for some animations, so be sure you have time to just leave that browser tab running while it does its thing...