[Zope] Check uploaded file for valid contents

Hello, I wonder if there is an elegant method to check a file which is intended to be stored as (Ext)File in the ZODB according to the validity of its contents. The reason is thatr I have to store some PDF files but I have users who do not really know the difference between DOC and PDF. I want to refuse DOCs (or any other non-PDF) to enforce users to upload just PDF. Kind regards ANdreas.