After the thumbnail is downloaded, an attachment for it is not created anymore. We just download the thumbnail to a specific directory we know to look for.
Replaced generic video icon with video thumbnail.
Cleaned up some code.
Removed figuring out which extension the thumbnail is because it is always a jpg.
Moved underscore templates to their own file.
Created helper functions to take care of extracting video ID from youtube URL and finding the path or url of the video thumbnail.
Changed the thumbnail of youtube videos on admin/upload.php to be the video thumbnail instead of WP's default video icon.