I don’t know if this is a browser issue or a MusicBrainz site issue. I am using FireFox 57.
When you click on Select images… when adding artwork a dialog box comes up for browsing to and selecting the image you want to upload. The file type selection defaults to All Supported Types.
What I see is this: All Supported Types (*.jfif;*.png;*.gif;*.pdf)
*.jpg is not in the list any more so I have to use All Files (*.*) to be able to select jpeg images. Although functionality is not totally broken it is irritating as default All Supported Types used to work fine.
MusicBrainz supported MIME types did not change recently and still include image/jpeg.
*.jfif is one of the file extensions associated to the MIME type image/jpeg but it is not the only one: *.jpg and *.jpeg should be recognized too.
Firefox might be using a native file select dialog box directly from your operating system (or desktop environment). It looks like a system setup issue to me.
I have looked at my registry and all the image/jpeg MIME type file extensions are correctly set.
I have also looked at the JavaScript and can see that the accepted MIME types and file extensions are defined. The specific extensions that are accepted are jpg, png, gif & pdf.
This is really looking like a FireFox 57 issue. It would be nice if someone else running FireFox 57 can confirm that they have the same problem.
I will install an older version of FireFox and see if the problem goes away.
It was a Windows registry issue. I found the correct key to change.
\HKEY_CLASSES_ROOT\MIME\Database\Content Type\image/jpeg\Extension was set to .jfif. I changed it to .jpg and now it works correctly. I have no idea what changed it to .jfif.
Chrome was working because it checks MIME types in a different order to FireFox. FireFox checks the registry first.
This .jfif issues is looking more like a Windows 10 Creators Update problem. I have just applied the latest Windows update and the registry key has reverted to .jfif.