Typefinding: differentiate between mp4 audio/video and quicktime etc.

Submitted by Stefan Kost @ensonic

Assigned to Stefan Kost @ensonic

Link to original bug (#340375)

Description

currently mp4 clips are detected incorrectly as "audio/x-m4a" in typefind. So, we would like to introduce new "video/mp4" mimetype for this purpose. The main point is that we need to distinguish the difference between clips with and without video content (since we have separate players for audio and video).