icat does not seem to scan dirs

(Running kitty 0.14.1.)

I ran `kitty +kitten icat dir-with-pngs` on both macOS 10.13.6 and Ubuntu 18.04 and got:

```Failed to open image: dir-with-pngs with error: identify: no decode delegate for this image format `' @ error/constitute.c/ReadImage/556.```

This seems to be against the documentation stating "Directories are scanned recursively for image files".