Creating Custom Bundle File Format

FROM : malcom
DATE : Thu Jul 20 00:07:05 2006

hi I've created a custom bundle file format (it should to convert a
folder with some resources in a 'fake' file). This is the screenshot
from xcode:

http://img411.imageshack.us/img411/9749/provawk8.png

However if I try to set the extension of a folder with that suffix the
folder still a folder and nothing has changed. How can I create a
bundle format?