Every now and then I like to save pictures, memes, and whatnot of the Internet. Usually they are standard format, like JPG or PNG, sometimes GIF. But then an odd thing came about. Pictures ending with the extension .JFIF. I would not of noticed except that my picture viewer of choice, XnView from https://www.xnview.com/en/ doesn't seem to handle the format very well. According to the documentation that came with Xnview, this program should support the .JFIF format.
I have to use Windows Photos to open them. But there is another option. Renaming the extension. Since it's just seems to be a format of JPG, renaming the extension from .jfif to .jpg does the job.
To rename extensions, you'll need to see them first. So type in File Explorer Options in search. The click on it.
Then click on the View tab and scroll down to Hide extensions for known file types.
Uncheck the box. Then Apply and OK. Now you should see the extensions on your files. Just right click on a file to rename it or highlight it and press F2 and just rename the .jfif to .jpg. Just click on Yes in the warning prompt box that pops up after.