I found on my HD an old file (2017) with name file "mystery wallet" and no file extension. In those times I was playing with so many different cryptocurrencies, mining software and hardware (making basically $0), but I'd like to double check what this is and whether it contains anything, before trashing it.

I added the .txt extension and opened it on my Mac and it's one long string of characters (2,012 to be exact) that start with:

QklFMQJD5E and it continues.

What could it be? I tried to rename it wallet.dat and tried to import it into Bitcoin core with no success.

For scammers, don't even waste time sending me DMs, I don't respond to any DM.