Have you ever seen a JFIF photo format?
 

My desktop has just begun refusing to save any photo in .jpg format, instead of making it a .jfif one, which no photo editor I have can use. Anyone know where I should start to fix this? Any ideas welcomed!

I did a quick search and found a couple of sites that address this issue. It looks like the "file associations" doesn't have a program associated with it. Have you tried opening the file with a different program? I have "Microsoft Office Picture Manager" that was included in my Office 2010 - but support ended for it Oct 13, 2020. It supposedly will open a jfif file (at least that is what my "file associations" said.

https://nspeaks.com/jpg-images-getti...es-how-to-fix/
https://filext.com/file-extension/JFIF

The last one has instructions depending on operating system via a drop down box. Which operating system do you have?
Have you tried right clicking and "Open With" and choosing a photo editor? I'm pretty sure that you can then set that program as the default.

I just tried to save a screenshot that I pasted into "Paint" as a jfif and it lists it along with jpg jpeg jpe In the past I have just changed the file extension of a file that I knew was an image file to jpg and opened it.

Janey - Neat people never make the exciting discoveries I do.
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Finally found some information that seems to have worked -- I found three YouTube videos on this subject. It is identified as a registry 'error' introduced by a recent Windows 10 update. I was able to follow one of the videos well enough to reset the file extension to jpeg, so going forward I should be ok. What a painful path this has been!