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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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))