Add MP4 to mime.types

Its a popular video format, we should recognize its type.

This will also prevent people from uploading mp4's to commons.

Wikipedia claims m4a sometimes used for video content.
RFC 4337 says audio/mp4 is mime type for mp4 with no
video (which is usually what an m4a extension means).

Bug: 52990
Change-Id: I5c24c3e318651241cced5b36d107d3603a289427
Merge "Add MP4 to mime.types"