While upgrading my Ubuntu from 7.04 to 7.10 to 8.04 I encountered some errors starting up Konqueror or opening a file chooser dialog. The popup would read "No MIME types installed", and there would be no files listed. Autocompletion on the input box would work though…

After looking around, blaming a KDE version upgrade, or configuration problems, or … I found that the /var filesystem had been full at one point in time, and while there was space now, something got corrupted.

Cleaning out /var/tmp/kdecache-<username> should solve the problem for you if you have it too. (No need to re-login, I think) 

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