In windows XP this was easy, in Windows 7, apparently one needs to edit the registry as suggested here:
Let XYZ be the extension you want to deassociate.
With a registry editor (e.g., regedit.exe),
1. go to
HKEY_CURRENT_USERSoftwareMicrosoftWindowsCurrentVersionExplorerFileExts
remove the subtree .XYZ
2. go to
HKEY_CLASSES_ROOT
remove the subtree .XYZ
remove the subtree XYZ_auto_file
When I tried this, step 1 seems to suffice. Why it has to be so complicated is beyond my understanding.