This implements a new VideoMimetype handler for all video/* files,

which provide a browser-supplied thumbnail for uploaded videos. This

handler will generate an HTML thumbnail that sets up a <video> tag

starting 0.5 seconds into the video, prompting the browser to load

enough of the video to display an early frame and to stop. The video is

set to not auto-play, to mute audio, and to preload only metadata along

with the frame.
When hovering over the thumbnail, the RB.FileAttachmentThumbnailView

will look for a <video> tag and play the video on loop, instead of

scrolling the frame. When the mouse moves away, the video will pause,

but retain the current position within the video for later resuming.
This does a lot to help video files not appear so blank. There is room

for improvement. For instance, if the video file cannot be played by the

browser, or it can only play audio but not the video, then the thumbnail

will appear blank. This is no different than it was before. Fixing it

would involve providing a poster="..." argument that points to an

image to display. However, for now, this is being left out, as this is

still an improvement over the existing behavior.

Tested with a bunch of videos (some that could be displayed, some that

could not). Verified that the browsers weren't downloading the entire

thing up-front.
Hovered over the thumbnail and saw it start playing. Moved away and

saw it pause. Hovered over again and saw it resume.
Tested in Chrome, Firefox, Edge, and Safari. Safari did not support

any of this for my test videos, but did not break.