Looks like this issue is extant again as of the
EPUB 3.3 spec. I had a book open in the editor that I'd manually coded some fonts in with MIME types 'font/ttf' and 'font/otf' and it failed the validation check. These are the new preferred values as of the adoption of
RFC 8081 in February 2017 and are now present in the EPUB3.3 resource table.