Have added these as valid file types in the options but they don't show in a
directory when browsing in the Gallery module. Webm does nothing, gifv flashes a
quick warning that "your browser doesn't support flash or HTML5" (I'm using
latest Chrome) which dissapears immediately.

I realise the front end template will need adjusting to properly embed these
images but currently it's not possible to manage them with the admin panel
module interface.