I don't know for sure since I have macOS High Sierra which supports HEIF but there is a setting under Settings->Photos->Transfer to MAC or PC->Automatic which is suppose to convert JPG when transferring to a computer that doesn't support HEIF. I've never tested it however.
I wish it worked like that. But, then, who knows what “other” setting or update in IOS or High Sierra undid or compromised my intent.
All I do know is, my “efficient” X photos were viewing fine thru iCloud download to MBP until mid Feb. Then I started getting an ! inside a triangle when tried to open in Photos.
the pik is fine in the thumbnails, fine in the strip to the left for a day’s series, but try to open one with a doubleclick for larger view and after a split-second view up pops the triangle instead.
The problem is worse now, going back further date-wise in my photos on MBP after Apple support said increase iCloud storage, be sure selected iCloud uploads, photo stream, etc.
Time to reopen that Chat, I guess. Really close to the point of downloading all photos to my TB drive and selling the MBP.
Note: I had transfer set to Most Compatible and the problem began while that setting was in place.
UPDATE: Chat reopened 6 am Texas time and Apple rep (1 pm Irish time) said a problem in the iCloud needed to turn over to Apple engineers, resolve 6-48 hours. Perhaps his Irish engineers pick it up today or probably awaits USA awakening. Here’s hoping this works!