assrender: Add "application/vnd.ms-opentype" mimetype detection
The "application/vnd.ms-opentype" mimetype is commonly used in many fonts attached in the matroska videos. Assrender should treat it as compatible without the need of parsing the file extension.
This kinda depends on !1967 (closed), bacause as of now the mimetype detection in assrender
does not work anyway. This can be merged, but will not take effect until mimetype detection is fixed.