I don't know exactly what you mean, but if you can use an OpenFileDialog instead of a FolderDialog you can set the Filter property to make it show only the files with a certain extension.

The filter property is a string that first takes the 'display' of the filter (which is displayed in the dialog) and then the actual filter, separated by a |. Here is an example:
Code:
OpenFileDialog1.Filter = "Text files (*.txt)|*.txt|Bla files (*.bla)|*.bla|All files|*.*"
This will show three options in the Filter combobox (in the openfiledialog):
"Text files (*.txt)" (if this is selected, only .txt files are shown)
"Bla files (*.bla)" (if this is selected, only .bla files are shown)
"All files" (all files are shown).