docstr: Returns file extension.
code: def extension(self):
      """Returns file extension."""
      return self._extension
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import os
  import datetime
  from yattag import Doc, indent
label_desc: Use image node_id 6 for calling the CustomHtml obj's underlying member method code with example usage: obj.extension() and returns: self