Windows 10 had been saving nearly every image as a '.jfif' extension since a recent update. Managed to fix it.

[PetterJhon]

Hello everyone,,
Since the creators update, for some reason, saving any jpg, and some other images, regardless of source, would save as a jfif file, which was cumbersome to deal with as while most apps would open it, they wouldn't by default. It took me several weeks to nail down the problem, I thought it was my web browser for a long time, but it turns out it was a Windows issue with the 'creators' update apparently, at least the thread that finally solved the problem for me blamed that over the web browser.

To fix,

HKEY_CLASSES_ROOT\MIME\Database\Content Type\image/jpeg

'Extension' was set to jfif and not jpg. Setting it to .jpg fixed the issue with all files from all sources and apps.

Posting this here as it took me a long time of googling to find the result and hopefully this helps some people.

Always backup your registry before editing.

[casanoffi]

I found this topic just because I was curious about your comment in the Switch area…

Really nice to know how to fix this issue, I was very distracted about the jfif file (never saw this kind of data)...