如何在codeigniter中覆盖具有相同文件名的file_ext

I want to replace/overwrite my image upload, which works. But the file have copied end up with another extension.

This is my controller

  function add(){ 

    if (isset($_POST['submit'])) {
            $id = $this->input->post('nisn', TRUE);

            $config['upload_path']          = './upload/';
            $config['allowed_types']        = 'jpg|jpeg|png';
            $config['max_size']             = 1024;
            $config['file_name']            = $id;
            $config['overwrite']            = TRUE;
            $this->load->library('upload', $

            $this->upload->do_upload('userfile');

            $upload = $this->upload->data();

            print_r($upload);
            die;
  }