This document provides you with an idea and a solution for restricting web access to a certain resource or location by specifying a custom useragent.
If you want to access a file on your webserver, but you also don't want it to be publicly accessible, then you maybe found out about http basic auth, but then you noticed that it's not so convenient to use with mpv and this is exactly where this solution comes and saves your day and future.
`mpv` has a config option that lets you specify a custom useragent: https://github.com/mpv-player/mpv/blob/master/etc/mpv.conf#L99