[Zope] Preventing download of Document object

Hi, I am customising a product (Document Library). Normally, when you select the URL of one of the Document objects the contents of the document (msword, pdf file etc) is downloaded by the web browser. I want to be able to limit access to the document so that, say, an anomynous user can view... www.myzopesite.org/doclib/Documents/doc/show_extract ....but not... www.myzopesite.org/doclib/Documents/doc ...where doc is an object of the Document class in question. I can use __ac_permissions__ to authorize what methods can/can't be accessed in the Document class but how do I enable certain methods whilst barring access to the file contents of the document itself? Regards, Simon B.