Use RFC 8081 font file MIME types in the EPUB builder (#11486)

The "font" top-level media type was formalised via :rfc:`8081` in 2017.
Standard MIME types for OTF and TTF fonts have been
`registered with IANA`_ as ``font/otf`` and ``font/ttf``.
These are also listed in the EPUB 3 `core media types`_ specification.

Sphinx-generated EPUB documents including custom OTF or TTF fonts
specify a MIME type for those fonts in ``content.opf`` of
``application/x-font-otf`` or ``application/x-font-ttf`` respectively.

The ``epubcheck`` application presents informational warnings that
these MIME types are non-standard when checking such a generated
EPUB document.
Adding the correct MIME type will prevent those messages
and will also result in a valid EPUB.

.. _core media types: https://www.w3.org/TR/epub/#sec-core-media-types
.. _registered with IANA: https://www.iana.org/assignments/media-types/media-types.xhtml#font

Author: picnixz

sphinx/builders/_epub_base.py

@@ -65,9 +65,9 @@
     '.svg': 'image/svg+xml',
     '.jpg': 'image/jpeg',
     '.jpeg': 'image/jpeg',
-    '.otf': 'application/x-font-otf',
-    '.ttf': 'application/x-font-ttf',
-    '.woff': 'application/font-woff',
+    '.otf': 'font/otf',
+    '.ttf': 'font/ttf',
+    '.woff': 'font/woff',
 }
 
 VECTOR_GRAPHICS_EXTENSIONS = ('.svg',)