[Zope] Indexing files

Hei All. I have the following problem: I am building up a ZCatalog and indexing my DTML methods. I use the index type ZCTextIndex and the object function PrincipiaSearchSource. It works fine. But when I try to index my Files (type File) with index type ZCTextIndex and the object function SearchableText it finds no words and the index is empty. Am I using the wrong object function? Thanks, Sune