go-camo was indeed originally intended to proxy only images, for two reasons: 1) the camo project it was inspired by only proxied images 2) my use-case at the time only required proxying of images
Later, a fork was created by a user to additionally proxy fonts and css. I wasn't comfortable including those in go-camo -- see discussion on https://github.com/cactus/go-camo/issues/20.
In my experience, video files are "usually" either linked (by url, no content warning), uploaded (service hosts it, so no content warning), or inlined from some hosting service (eg. youtube, vimeo; ssl provided by service). Video files are also generally much larger than image content.
Can you further describe your use-case/requirements for proxying video?