If the extension you want to change is in the framework, check the
IDR_MAINFRAME string in your string resources. It might look something like:

Test\n\nTest\n\n\nTest.Document\nTest Document

Change it to something like:

Test\n\nTest\nTest Files (*.xxx)\n.XXX\nTest.Document\nTest Document

where XXX is your desired extension.
If it is the extension of a file dialog you pop up yourself, you put the
extension like:

CFileDialog dlgFile(TRUE, "XXX","*.XXX",OFN_HIDEREADONLY |
OFN_OVERWRITEPROMPT,"Test Files (*.xxx)|*.xxx||");

Johan Rosengren
Responsable Informatique
PACTA S.A.