def process_tree_recur(self, file_names, node):
        """
        Adds the names of all the files associated with the sub-tree rooted by `node` to `file_names` in post-order.
        :param file_names: A global list containing all file names associated with a tree
        :param node: The root of the current sub-tree being processed
        :return: None.
        """
        # Process node's children
        for child_node in node.children:
            self.process_tree_recur(file_names, child_node)  # Call children first in case a tiled thumbnail is needed

        file_names.extend(node.process_files())
Adds the names of all the files associated with the sub-tree rooted by `node` to `file_names` in post-order.
        :param file_names: A global list containing all file names associated with a tree
        :param node: The root of the current sub-tree being processed
        :return: None.