Returns a list of the file tokens in the list of parts.

def file_parts(self):
        """ Returns a list of the file tokens in the list of parts. """
        file_parts = []
        for part in self.parts:
            try:
                for sub_part in part:
                    if isinstance(sub_part, FileToken):
                        file_parts.append(sub_part)
            except TypeError:
                if isinstance(part, FileToken):
                    file_parts.append(part)
        return file_parts