I was looking for a specific Acorn image, which I couldn't remember the filename of but I did remember the folder it was in as well as the text in it ("fx"). And then I also remembered that text boxes in Acorn files are indexed via Spotlight, and since I was already in Terminal I ran the following command in that folder:

$ mdfind -onlyin . fx
stylechain-layer@2x.acorn
stylechain-layer.acorn
stylechain-layer-highlight@2x.acorn
stylechain-layer-highlight.acorn
…

And tada, there were the images I was looking for.

Doing this in Terminal is not necessary, I could have also easily done this via the Spotlight menu.