code: public function setIsMultiple($flag)
      {
          $this->isMultiple = (bool) $flag;
          if ($flag) {
              $this->setAttribute('multiple', true);
          } else {
              $this->removeAttribute('multiple');
          }
  
          return $this;
      }
docs: Sets if this element allows multiple files to be selected.
  
  @param boolean $flag
  
  @return self