custom standalone files

Hi,
I am in the process of building a standalone app. I would like to be able to use the 'save as' menu in my application to save the project/file in my own format (.myformat instead of .mxb etc). I have seen this done (King tools), but I couldnt find any reference about how to do it in the docs/help (standalone object) or in the forum.Any help is greatly appreciated. Thanks
dan