Add support for the .opus file format

The codec and container format are already supported, but not in that
combination. Simply add the file extension as a type of Ogg file (which
it is).

Change-Id: Ia9134a8d1205f73da12f579b3ad62367b2b8d88e
See: https://issuetracker.google.com/issues/37054258
(cherry picked from commit 32a5b165)