https://community.metabrainz.org/t/edit-coverart-types-to-have-multiple-types/437363/7

Basically Add and Edit Cover Art pages are using different UI to set and to modify cover art types.
One is using a list of checkboxes, the other a dropdown list (together with usual Shift/Control/whatever to select multiple types).

It should be unified, checkboxes are more obvious for most users.