"Sigil now uses "application/x-font-ttf" as the media type in the OPF for TrueType fonts (issue #609) "
It's not critical, but it seems that the IDPF is moving towards standardising "application/vnd.ms-opentype" for both .otf and .ttf files. See
http://www.daisy.org/epub/issues/mis...ype-font-files.