this is highly unlikely to be a Photoshop issue since the opening of files is controlled by the Mac's Finder.
the OSX forums are the place to look for help on this
you may have to delete the finder p-list. [google is your friend here]
here's some info about that: Finder issues? How to delete its preferences
be aware that such actions within the guts of OSX as deleting p-list files can cause issues, you must be sure you have a good backup before messing with preferences files.
proceed at your won risk.
you'll probably find, though, that the the next update to High Sierra fixes it.
High Sierra's a VERY new software release at this time and lost pf issues are being reported.
I hope this helps
if so, please do mark my reply as "helpful" and if you're OK now, please mark it as "correct" below, so others who have similar issues can see the solution
neil barstow, colourmanagement
Wow, so glad I didn't mess with my preferences. Nothing personal, but that would have aggravated me to no end. Plus, the link you referenced is quite old. Been working on Macs since 1990 (System 6?), so I was bewildered but not reckless. May have found the culprit after all. It seems that recent updates to Firefox/Mozilla have been causing image downloads to revert to Preview. Once I downloaded something, a Google image, for instance, everything blew up from there. My OS decided, with each Firefox update, that all images should then be opened with Preview because Firefox said so. (Since when does Mozilla tell my computer what to do?) That explains why it was so randomly intermittent. Go figure. Changing the file management settings in Firefox preferences has apparently fixed the issue. We'll see. I'll report it to Mozilla and see if they've got an explanation. Fingers crossed it stops happening. I'll be testing the theory a bit more when I'm between deadlines. - - Sue R
In Firefox Preferences: General > Applications > Content Type > find image formats such as jpeg, tiff, gif, etc., and load Photoshop into the pulldown option.