Perhaps it's already possible and I have something misconfigured but when I double click in macOS's Finder on a .fits file then KStars itself opens (configured that well) but doesn't open the FITS viewer with the selected file. Would that be possible to configure/setup?
Thanks for the great work and port (Stephane Lucas) to macOS!
AFAIK, this is working on Linux so for MacOS/Windows, KStars should register with which meta type it can read/open... though even in Linux it will always start a new process now whenever you click on a FITS file instead of re-using the existing process.
While this might not be helpful in solving this particular issue with opening the images in the KStars preview window, the solution I use is Observatory for the Mac. codeobsession.com
It installs a fits/xisf plugin for preview functionality in the finder. So not only will it turn your image thumbnails into images, but you can also press the spacebar to preview the image in a large window.