The file "" typically refers to a Rocket League Workshop map or a specific compressed asset package used in game modding . Because it is a .7z file, you must first extract its contents using a program like 7-Zip or WinRAR .
If this file contains a Rocket League custom map (usually a .udk file inside), use this method for or Steam versions:
Restart RetroArch and go to to verify the new system is listed.