Video: Fails with 500 if not specifying .mp4 extension

Example failing URL `graph/passthrough?input=https%3A%2F%2Fs3-us-west-2.amazonaws.com%2Fcdn.thegrid.io%2Fassets%2Ffeature_color.720x450.mp4`

This happens because when parsing the request, we always default to `outtype='.jpg'` - which currently fails when the `input` is a video file (ref #109).

Related to #32