Manual:$wgMimeDetectorCommand

Details
Sets an external MIME detector program. The command must print only the MIME type to standard output. The name of the file to process will be appended to the command given here.

If this setting is not set, the PHP fileinfo extension will be used, or if that is unavailable, the PHP function  will be used.

It is important to note that MediaWiki itself performs a set of MIME detection tests itself, and only if those are unsuccessful is the MIME detector invoked. See Manual:MIME type detection for more information about this whole process. See also Manual:Configuring file uploads.

Example (Linux): or shortly,