It happened exactly one time to me, that I joined a server and a window was popping up, saying 'this is an optional file - do you wish to download it? (size x kb)'. I could then click and choose.

I wonder if that's a standard feature for servers or a special modification.

It wouldn't help us lot at the moment because all files are necessary for the maps, but maybe it's possible to put some optional content in maps without causing a problem between the players.
For example a bunch of optional skin-overlays that could come with a map. Those who like could download it and the other ones wouldn't see a difference…

Just curious.

Do you want to download the optional package "autoaim" ?
Y/N