davesmith87 A .key file is a text file so you could rename to .key.txt. Or compress to a zip file as we have had to do forever. I agree, it would be nice if the file picker wasn't so picky.
Because the key file is text, you could copy and paste into any one of a number of notes fields, however you are tracking the certificate.
Best practices is a worthy and lofty goal, but doesn't always take into account what has to happen to get the job done.