The ImageMimetypeTests class has a setUp() method, but it didn't

call the parent class one. I discovered this while running tests via

pytest-xdist, and hit failures in this class because it was the first

set of tests running on that thread, and initialize() had not been

called before we were trying to access the review UI registry.

Ran unit tests with xdist and a number of threads that exposed this bug,

saw everything succeed.