This feature is a client and server side option.
Lets say for example that a server is using a map that requires the c2nite_ skybox textures. If the server doesn't have those textures, then a random client that has the textures will upload them to the server, then the server will send the textures to the clients that don't have them.
Server admins can choose if they want to allow uploads from clients or not.
Players can choose if they want their copy of Paintball 2 to upload their files to the server or not.