I believe "autoopen" is specified by the registry key "EditFlags" underneath the registry entry for the file type.

For instance, HKEY_CLASSES_ROOT/.pdf's (Default) points to AcroExch.Document. If you open HKEY_CLASSES_ROOT/AcroExch.Document, you should be able to add a REG_BINARY key called "EditFlags" with the value "00 00 01 00".

So the key then is figuring out what file type it's mapping to ... Obviously you can check .zip first to see ... You may want to check the MIME Database in the Registry and see if that maps to something interesting too ....