As far as I can tell, the EPUB 3 spec only specifies the mimetype for otf files, not ttf files. In fact if you read
http://www.idpf.org/epub/301/spec/ep...re-media-types ttf files are apparently not a core media type in epub 3.
So if epubcheck is complaining about any particular mimetype for ttf files being wrong, epubcheck is wrong. As far as I can tell, the spec has absolutely nothing to say about what mimetype should be used for ttf files any more.