That function doesn't do what you think it does.

To get path to the file in the tracker that sotres
file contents use: self.db.filename as below (from hyperdb.py):

    def export_files(self, dirname, nodeid):
        """ Export the "content" property as a file, not csv column
        """
        source = self.db.filename(self.classname, nodeid)

        dest = self.exportFilename(dirname, nodeid)
        ensureParentsExist(dest)
        shutil.copyfile(source, dest)

exportFilename is used to generate the *target* destination
path for the exported directory.

What you want is self.db.filename given the class and nodeid.