I'm using Debian Jessie 64-bit with Gnome 3.14.1.
My TIF image files are opening by default in the PDF Viewer, don't ask why. In Nautilus, I right-click on the file, click Open With|Other Application...
If I right-click "Image Viewer", there is only "Forget association".
Where is the "create default association" button?
I've found a ~/.config/mimeapps.list, which contains
It seems that something else is overriding this configuration.
It seems to be /etc/gnome/defaults.list, which contains
which is the PDF viewer in question. Removing this line solved this problem. The problem remains, though, if there should be a button "add default" to the image above.