Comment 1 for bug 1365668

 https://forum.kde.org/viewtopic.php?f=66&t=118893

There is the same Problem described, a lot icons do not show up anymore, at my case it was from most installed Programs.

Post #4 gives the solution

-> try (as root and anyway)
gdk-pixbuf-query-loaders --update-cache

It seems this command was not implemented into the upgrade system and caused the broken Icons.
Please add it for the upgrade process.