Improve test coverage of the `betty.media_type` module

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.84%. Comparing base (6c4cf03) to head (76fdc54). Report is 1 commits behind head on 0.4.x.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.4.x #1919 +/- ## ========================================== + Coverage 96.82% 96.84% +0.01% ========================================== Files 259 259 Lines 18310 18338 +28 Branches 3475 3485 +10 ========================================== + Hits 17729 17759 +30 + Misses 392 391 -1 + Partials 189 188 -1 ```

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