Cannot change KDE file application preference

[Axel G. Rossberg]

I am using okular on a gnome desktop. In a PDF file I am viewing with okular, there are embed links of the form "file:///<path>/<filename>.gif . Until recently, okular opened my web browser firefox when clicking on the links. Now it opens the ImageMagick display program. Unfortunately, ImageMagick cannot handle my compressed, animated GIFs at all. I'd like to change the associated application to firefox or the gnome image viewer (works like a charm), or at least the application called by xdg-open (currently this is gnome image viewer).

So, I go to the File -> Open dialogue of okular, chose a .gif file, do right-click -> properties, click on "File Type Options", and try all kinds of things to remove ImageMagick as the first option (Remove, Move Down, Move Firefox up). Then I press OK.

An "Updating System Configuration" window briefly pops up, but the status bar does not grow very large (stays at about 5%). Then the window closes.

Then I click "File Type Options" in the Properties dialogue again.

Expected result: my requested changes have been made.

Actual result: nothing changed, ImageMagick is still preferred application for .gif files.

Fix of the bug welcome, but also hints as to which config file to change by hand for this.

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: okular 4:15.08.2-0ubuntu1
ProcVersionSignature: Ubuntu 4.2.0-18.22-generic 4.2.3
Uname: Linux 4.2.0-18-generic x86_64
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
CurrentDesktop: Unity
Date: Mon Nov 30 14:16:01 2015
DistributionChannelDescriptor:
 # This is a distribution channel descriptor
 # For more information see http://wiki.ubuntu.com/DistributionChannelDescriptor
 canonical-oem-somerville-precise-amd64-20120703-2
InstallationDate: Installed on 2013-08-01 (850 days ago)
InstallationMedia: Ubuntu 12.04 "Precise" - Build amd64 LIVE Binary 20120703-15:08
SourcePackage: okular
UpgradeStatus: No upgrade log present (probably fresh install)