Allow WEBM Uploading

It would be nice if screencast allowed the upload and hosting of .WEBM videos.  This was possible up until Oct 2021 but this capability was revoked.  Despite using Camtasia we have to use handbrake to convert to WEBM so that html5 video element in Chromium works (MP4 is not natively supported by Chromium).  We can no longer use screencast and are forced to use another provider.