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. Testing Done: Ran unit tests with xdist and a number of threads that exposed this bug, saw everything succeed.