Qt 6.7.3 seems to have switched to a different embedded MIME type
database. This one returns `application/x-font-{otf,ttf}` for OpenType
& TrueType fonts. MKVToolNix did not know about these two types &
therefore used them as-is, not mapping them to the well-known legacy
types if enabled, but not using the currently specced types
`font/{otf,ttf}` either.
This change forces mapping of all three known font MIME type types to
either the legacy or the current specs ones, depending on the "enable
legacy font MIME types" settings.
Partially fixes#3751.