Ah, the logic to load the initial database is with known types, common types and the list of files from mimetypes.knownfiles subsequently overriding each other. At the start of the test mimetypes.knownfiles is set as empty list but since I am using script_helper.assert_python_ok setting the list of knownfiles to empty list doesn't work and mac seems to load a mimetype for .pic that is different from common types and overrides it.

I guess my tests can be rewritten to make sure the tests are not machine dependent based on the files.

I have moved the code for __main__ to be a private function to help testing by directly calling it instead of using subprocess to invoke python -m mimetypes. I have also found that mimetypes.knownfiles was set as empty at the top of the module. It has a potential side effect that all other tests that follow after test_mimetypes will also have empty list of files when accessing mimetypes.knownfiles. I have moved it to setup call so that this change doesn't affect other tests.