File catalog, URL field, why gimp opens pdf files?

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Version: 1.3.2.1 3.5.10 Kubuntu (intrepid) 4:3.5.10-0ubuntu6
OS:           Linux (i686) release 2.6.27-9-generic
Compiler: Target:i486-linux-gnu
Severity: Normal

Below is the question.

The system is Ubuntu Desktop 8.10.
I created a file catalog.  There is a URL field. The target is a pdf
file such as file:///location/xxx.pdf. When I click the URL, GIMP
program is started.
I wish it is not GIMP but evince, adobe reader or pdf editor.  However I
cannot find out a solution. Is it a bug? What should I do? Could you
please tell me? Thanks!