HTML source type for video is hardcoded as video/mp4

[yugokato]

The HTML source type for videos is currently hardcoded as "video/mp4" in the jinja2 template. This seems to prevent Safari from playing webm video files, although Chrome and Firefox seem to handle this fine. (NOTE: I use --self-contained-html option)

I've modified it to "video/webm" in the generated report and confirmed that Safari can play webm videos.

Please change the template to reflect the mime_type value given to extras.video().