Does the job but could be greatly improved

The UI is unclear. There are greyed-out checkboxes beside the types which are recognized by WordPress already. You can NEVER enable or disable them, so showing disabled checkboxes beside them is confusing, as it implies there’s some way to enable them, but there isn’t. Just LIST the built-in types, please, without implying that they can be edited. Also label them as the built-in types.
Checking a box beside something is generally considered to mean that you’re enabling it. Here, checking a box means you want to delete it. Counterintuitive. Please use a red-X or minus control rather than a checkbox to mark things for deletion.
It would be VERY HELPFUL to have a built-in list of the more common unrecognized filetypes that could be enabled with one click or check, rather than having to look up the MIME type online every time. epub, mobi, and azw formats in particular.