This sounds like this:
The RIAA attorneys will try to download copyrighted materialfrom your IP and if successful, will offer a settlement or threaten to sue because you made copyrighted material available for others to download (which means you uploaded it to them).
But if there was only one file, it’s hardly worth their time. If this is them, you probably had a bunch of files available and they will want something like $750 for each song you have in that upload folder.
Thank you