removed the ability of the project to update the cached video object
You should create a new instance of project to update the vide, it's
simpler and works the same way for most cases.
now we do not use an extension whitelist for video_thumbnail
since videothumbnail is downloaded using vimeo/youtube api and we do not
have to check for a file extension I have removed the whitelist to avoid
problems with the lack of a proper filename due to open-uri