we try to not insert external files in unit tests, and rather we should use mocks.

utils.js is a special case (see that we still use a mock_utils.js).

Here, I'd advise to do a mock_mime_mapper.js where functions would return something like the concatenation of their arguments, and you would just need to check the filename are what you expect in your test.

If you properly add unit tests for mime_mapper.js, then everything will be covered !