After conducting research, it appears that "fg-optional-mp-files.bin" is a binary file associated with the Forza Gaming (FG) platform, specifically with the Forza Motorsport series. The file is likely a container for optional, multi-player related data.
Rename it to fg-optional-mp-files.bin.old instead of deleting. This allows you to restore it if something unexpected happens. fg-optional-mp-files.bin