code: public function setExtension( $extension )
      {
          if (strlen( $extension ) > 5) {
              throw new InvalidArgumentException( sprintf( 'Extension should be no longer than 5 characters (%s)',
                  $extension ) );
          }
  
          $this->extension = $extension;
  
          return $this;
      }
docs: @param  string $extension
  
  @return $this