[Zope] webdav: collection does not work when I use file type object as baseclass

Hi there, I am creating a product that has a folderish and a file type aspect. I therefore use both ObjectManager and DTMLMethod as its base classes. When I do that, I can not use Webdav to access folders which contains instances of my product anymore. If I get rid of the DTMLMethod as base class things work fine again. Is such a combination of base classes sensible? If yes, what must I do to have webdav working? thanks for your help. Robert www.redcor.ch