Fix MIME type for TrueType fonts in EPUBs (#6464)

Per the EPUB 3.2 spec, "application/x-font-truetype" is no longer a
valid identifier for TrueType (.ttf) fonts [1]. This fixes warnings when
validating pandoc-generated EPUBs using `epubcheck` [2].

References [3].

[1]: https://www.w3.org/publishing/epub3/epub-spec.html#sec-core-media-types
[2]: https://github.com/w3c/epubcheck

src/Text/Pandoc/MIME.hs

@@ -461,7 +461,7 @@ mimeTypesList =
            ,("ts","text/texmacs")
            ,("tsp","application/dsptype")
            ,("tsv","text/tab-separated-values")
-           ,("ttf","application/x-font-truetype")
+           ,("ttf","application/font-sfnt")
            ,("txt","text/plain")
            ,("udeb","application/x-debian-package")
            ,("uls","text/iuls")