docs: def parse_render(render):
      """Parse render URL parameter.
  
      >>> parse_render(None)
      'png'
      >>> parse_render('html')
      'png'
      >>> parse_render('png')
      'png'
      >>> parse_render('jpg')
      'jpeg'
      >>> parse_render('gif')
      'gif'
      """
      formats = {
          'jpeg': guess_all_extensions('image/jpeg'),
          'png': guess_all_extensions('image/png'),
          'gif': guess_all_extensions('image/gif'),
          'bmp': guess_all_extensions('image/x-ms-bmp'),
          'tiff': guess_all_extensions('image/tiff'),
          'xbm': guess_all_extensions('image/x-xbitmap'),
          'pdf': guess_all_extensions('application/pdf')
      }
      if not render:
          render = 'png'
      else:
          render = render.lower()
          for k, v in formats.items():
              if '.%s' % render in v:
                  render = k
                  break
          else:
              render = 'png'
      return render