[Zope] Instanciate a Zclass with "File" inheritance

How can I do to automatically instanciate a Zclass that is a subclass of "File" with DTML ? trying <dtml-call "REQUEST.set('file','c:\test.doc')"> brings the value of file as a string object and not as a file object What can I do ?? thanks christophe bec cbec@capgemini.fr