Starting with Moodle 2.9 we have a UI to add new mimetypes - see
But we still get bugs asking for new mimetypes to be added to the default set - see MDL-49243.
We need to decide what we do with these requests.
Any new mimetype request will be automatically accepted given it's available in the IANA list OR provides some verifiable value to the - learning - experience. Integrators reserve the right to decide, ask, vote... in controversial cases not covered by the previous sentence.