Thanks.
I changed the .xl (As you said, the file did open.) to .xls, and my desktop looks familiar again.
I'm at a complete loss as to what happened. The touch on my 2015 MBP's keyboard is so light that running a dust cloth across it frequently results in files/folders being selected and having their names either deleted or changed to odd character combos, but deletion of that "s" would have required a click...impossible, because my trackpad doesn't respond to pressure on the cloth.
