code: def get_compression_extension(self):
          
  
          build_request = BuildRequest(build_json_store=self.os_conf.get_build_json_store())
          inner = build_request.inner_template
          postbuild_plugins = inner.get('postbuild_plugins', [])
          for plugin in postbuild_plugins:
              if plugin.get('name') == 'compress':
                  args = plugin.get('args', {})
                  method = args.get('method', 'gzip')
                  if method == 'gzip':
                      return '.gz'
                  elif method == 'lzma':
                      return '.xz'
                  raise OsbsValidationException("unknown compression method '%s'"
                                                % method)
  
          return None
docs: Find the filename extension for the 'docker save' output, which
          may or may not be compressed.
  
          Raises OsbsValidationException if the extension cannot be
          determined due to a configuration error.
  
          :returns: str including leading dot, or else None if no compression