[Zope] How to use methods from OFS?

Hi! I'm trying to import and use OFS.content_types.guess_content_type(), Zope's method to get content types of files by name or content, in a Python script within Zope. But all I get is an Unauthorized error: "You are not allowed to access content_types in this context" (even Proxy role "Manager" didn't succeed). I guess invoking OFS modules needs special permissions or even External Methods. Is this correct, and can somebody give me a hint/a working example how to achieve this? Thanks a lot! And good night now... Martin