I believe that vnd. mimetypes do need to be registered. If you don't want to register, I think it would be video/x-gifv+mp4. http://tools.ietf.org/html/rfc2045#

I believe that vnd. mimetypes do need to be registered. If you don't want to register, I think it would be video/x-gifv+mp4. http://tools.ietf.org/html/rfc2045#page-13 http://tools.ietf.org/html/rfc2045#page-13