It's happened to every music producer at some point. You end up with some random midi files on your hard drive that you can't remember where they came from but they are kinda fire, but you're unable to find the key it's in as you're still learning music theory.
Midi Ai's free midi keyfinder tool is here to fix that.
First navigate to Midi Ai key finder page, once here you can go ahead and upload any Midi file you'd like by clicking the "Choose File" button.
Once your midi file has been loaded go ahead and click "Find Key", give it a couple seconds at most and then you should get a message saying what key the midi file is in.
That is how to use Midi Ai's free midi key finder tool