How do I use my hot keys to insert a picture into a fb comment?

Twice now over the past few months I’ve been typing in a comments field (NOT the original posting) and accidental keyed something that brought up the pictures files I have stored on my computer. It would appear that I could post a pic directly into the post, but….I’ve lost it both times before I could try it.
I think it was alt/something, but for the life of me I can’t replicate the key stroke. Ctrl/something? Ctrl/Alt/something? Hep me!

Using Windows and Chrome, although the first time I did it I was using firefox.