code: def _verify(self, payload):
          
          if not payload:  # Check payload has nonzero length
              raise ze.ParamNotPassed
          for templt in payload:
              if os.path.isfile(str(self.basedir.joinpath(templt["filename"]))):
                  try:
                      # if it is a file, try to open it, and catch the error
                      with open(str(self.basedir.joinpath(templt["filename"]))):
                          pass
                  except IOError:
                      raise ze.FileDoesNotExist(
                          "The file at %s couldn't be opened or found."
                          % str(self.basedir.joinpath(templt["filename"]))
                      )
              # no point in continuing if the file isn't a file
              else:
                  raise ze.FileDoesNotExist(
                      "The file at %s couldn't be opened or found."
                      % str(self.basedir.joinpath(templt["filename"]))
                  )
docs: ensure that all files to be attached exist
          open()'s better than exists(), cos it avoids a race condition