This sends the following header to a remote media server:
Referer: minetest://<server_name>:port
This was verified with CTF and the Minetest Public Remove Media
server. If the servername was a plain IPv6 address it will
contain `:` characters and will be encapsulated in `[]` to
be a valid URI.
The fallback code shouldn't be needed and is a remnant of the GET
method that old media servers use. Clients using it are likely
to just waste bandwidth and having to download the media again
through the normal transfer from server method. The most reliable
method is to get all missing textures therefore from the server
directly and not spam the remote media server with 404s.
