/usr/share/mysql/magic is in percona-xtradb-cluster-server-5.7 5.7.20-29.24-0ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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# Add the following to the end of your /etc/magic file to get the 'file'
# command to recognize some MySQL files.
#
0 beshort 0xfe01 MySQL table definition file
>2 byte x Version %d
0 belong&0xffffff00 0xfefe0300 MySQL MISAM index file
>3 byte x Version %d
0 belong&0xffffff00 0xfefe0700 MySQL MISAM compressed data file
>3 byte x Version %d
0 belong&0xffffff00 0xfefe0500 MySQL ISAM index file
>3 byte x Version %d
0 belong&0xffffff00 0xfefe0600 MySQL ISAM compressed data file
>3 byte x Version %d
0 string \376bin MySQL replication log
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