[Zope] Preventing download of Document object

[Simon Blandford]

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.