code: def _output_file(self, frame):
          
          if self.buff:
              return self.buff
          elif self.multifile:
              return self.file_root + "_%03d" % frame + self.file_ext
          else:
              return self.filename
docs: If filename was used output a filename, along with multifile
          numbered filenames will be used.
  
          If buff was specified it is returned.
  
          :return: Output buff or filename.