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.