Optional server downloads

[Metalfist]

Recently I saw a server that had optional download files.
So you would connect and it would show: "downloading optional skinfileX - (press f10 to skip)"
and if you skipped you could play anyways, but you couldn't see the skin.

It was on the server 77.111.184.233:7777
their website: http://nemethalozat.dyndns-ip.com:8000/news.php

So my question, how do you set that on a server?
or would it be a mod?
Anyway I'm kinda curious about this, bcs I haven't seen it done before.

[Sp0ngeb0b]

This can only be done for mods/packages you are compiling, not for already existing ones.
PackageFlags is the keyword you have to look for. Simply create a .upkg file in your classes folder of your project and modify it for your needs