Well, I didn't have that file in that location, but I did find a octet-stream.desktop in /usr/share/mimelnk/application/. I tried renaming it, but it didn't help. I tried copying it to ~/.kde/share/mimelnk/application/ (there wasn't an application folder in mimelnk there before so I created one), though that didn't help either. Starting up k3b gives me two missing octet-stream error messages and then another message saying 'error: no mime types installed'. Is there a mime type package or something that apt-get upgrade missed when it did the upgrade?
EDIT: OK, I've fixed the problem by doing this:
and then rebooting. I'm not exactly sure deleting the directories did it or the rebooting, but it's fixed.