Currently saving files in Gwenview 15.12.3 defaults to using the .jfif extension for JPEG files. While this extension is correct, it is also seldom used for many tasks, including webpage images. Wishlist request for changing the default extension for JPEG files to .jpg or .jpeg. Reproducible: Always
Please open a bug in your distribution's bugtracker. The file extension is not specified by Gwenview, it comes indirectly from the shared-mime-info database (/usr/share/mime/packages/freedesktop.org.xml etc).