New functionality for the lay user, most importantly a way to define lists of MIME types equivalence groups via the admin UI.
Such default groups are provided and can be further managed as configuration.
Refactored the code, excluding the extension validation (we trust core to do this).
This allowed to correct the bug reported in #2861255: [D7] Refactor the extension<=>mime_type logic.
The main check that the module does now is whether the mime-from-extension detection agrees with the mime-from-fileinfo detection.
Also added some unit tests.