I searched here and did not find anything on this. I tried to use an extension [.jfif] image as album art, and Mp3Tag did not see it in my Cover Art Folder under Add Cover. However, when I simply renamed the image with extension [.jpg] which is essentially the same as [.jfif] (so I'm told), MP3Tag saw it with no problem and applied it to my mp3 file. Thanks.
The id3 standard suggests jpg and png files for best compatibility. Any other picture format can cause trouble. So I would transform that other format to one of the better supported ones.
MP3tag eases the way to select the good ones.
You can find any other file if you enter
*.jfif in the file selection dialogue.
And: just changing an extension of e.g. a picture file to txt, it does not become a poem.
I do not see any buggy behaviour here.
JFIF is JPEG for all practical intents and purposes.
Added with Mp3tag v3.00a.
Thanks kindly. Look forward to the auto-update when available.