Support special video/3gp file.
Support special video/3gp file.

A 3gp video made by Blackberry 8900, it has 'major_brand: 3gp4, minor_version: 256, compatible_brands: 3gp53gp4isom'.
Yes, the 'isom' in compatible_brands.
So, it should check compatible_brands also.
h2non/filetype.py