Returns a list of filenames sorted by loading order.
    Only includes files that Loader's standardized loading knows how to load

def GetLoadingOrder(self):
    """Returns a list of filenames sorted by loading order.
    Only includes files that Loader's standardized loading knows how to load"""
    result = {}
    for filename, mapping in self._file_mapping.iteritems():
      loading_order = mapping['loading_order']
      if loading_order is not None:
        result[loading_order] = filename
    return list(result[key] for key in sorted(result))