The application/xhtml+xml MIME type should appear in the mime.types file and
similar (magic) files, associated with the ".xhtml" and ".xht" file extensions.
See RFC 3236 for more information, plus the Shared MIME-Info Spec at
Steps to Reproduce:
Type "file -i my-xhtml-file.xhtml" from a shell prompt.
file: Using regular magic file `/usr/share/magic.mime'
It's in current files.