docs: def create_transient(self, input_stream, original_name, length=None):
          '''Create TransientFile and file on FS from given input stream and 
          original file name.'''
          ext = os.path.splitext(original_name)[1]
          transient = self.new_transient(ext)
          if not os.path.isdir(self.transient_root):
              os.makedirs(self.transient_root)
  
          self._copy_file(input_stream, transient.path, length=length)
          return transient