Add file content from aff4_object to result.

def _ExportFileContent(self, aff4_object, result):
    """Add file content from aff4_object to result."""
    if self.options.export_files_contents:
      try:
        result.content = aff4_object.Read(self.MAX_CONTENT_SIZE)
        result.content_sha256 = hashlib.sha256(result.content).hexdigest()
      except (IOError, AttributeError) as e:
        logging.warning("Can't read content of %s: %s", aff4_object.urn, e)