You have to put the rfcmp in the packages folder and set to True the Enable Autodownloads parameter in the Multiplayer.json "Enable Autodownloads":true, "Enable Autodownloads#":"Whether to allow clients to autodownload files that they are missing.",
proba.rfcmp and proba.rfmod is another files. Not track or car. I create a rfcmp file to track and car? The car and track is in the installed folder.
There is a site on Steam, that has all the server's downloads isn't there? It was MUCH faster, as I recall, than trying to get a mod from the host server.