Please describe the problem:
In the file properties, open with tab there are mostly multiple selections for opening a certain fyle type. But is is not possible to select an other one than the one that is allready marked. 

Steps to reproduce:
1. Right click on a pdf file (or any other fyle type)
2. Click on properties and then open with
3. Click on another choice than the default one


Actual results:
I want to use Acrobat Reader for opening pdf files but in Ubuntu Dapper "documentviewer" is marked as the standard choice. 
 When i click on the circle in front of Adobe Reader it doesnt mark black

Expected results:
change the default choice into another one

Does this happen every time?
yes

Other information:
This could of course be related to my distribution? (Ubuntu Dapper)