/usr/share/doc/php-mime-type/MIME_Type/docs/examples/custom-magic.magic is in php-mime-type 1.3.1-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | 0 search/1/c =<?php PHP script text
!:mime text/x-custom-mimetype
|