Update MediaCrush support

This makes a few changes:

- Rely on blob_type instead of user-supplied mimetype to figure out
  media type (i.e. video/audio/image)
- Set flags (like autoplay/loop) according to user-specified values.
  This is valuable, for example, if someone uploads an actual video and
  sets the flags so that it behaves like a GIF (autoplay+loop+mute).
- Fix issue with link on branding for albums