Servers can distribute custom maps and assets to players so there's a mechanism to download files to a users computer. As for uploads, players used to be able

Servers can distribute custom maps and assets to players so there's a mechanism to download files to a users computer. As for uploads, players used to be able to set custom models in quake 2 which were distributed to other players on the server. Though I am not sure if that was done by server admins in special cases for clan payers or members or if there was an actual upload mechanism in the game engine.