This file is indexed.

/usr/share/doc/postfix-mysql/README.Debian is in postfix-mysql 2.9.1-4.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

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The postfix-doc package contains documentation on how to configure this
map type.  See /usr/share/doc/postfix/html/MYSQL_README.html