modal untuk upload image

Salam kenal para master Codeigniter Indonesia. Saya baru belajar codeigniter dan untuk latihan kasus saya buat aplikasi, didalam aplikasi tsb ada fitur upload image dengan menggunakan form_open_multipart yang terdapat dalam modal. Kendala yang saya hadapi adalah: Ketika form_open_multipart tidak di beri attribut id, contoh:(<?php echo form_open_multipart('berita/insert')?>) maka proses upload dan simpan data berjalan normal walaupun ada sedikit masalah tp bila form_open_multipart di beri attribut id,contoh:(<?php echo form_open_multipart('berita/insert',array('id'=>'berita_add'))?>) maka muncul error "no image file selected" dan tidak melakukan proses upload maupun simpan data, dibawah saya sertakan sedikit codingan yang sudah saya terapkan: //**view**//

 <?php
defined('BASEPATH') OR exit('No direct script access allowed');?>

	<div class="modal fade mymod" id="add_berita" role="dialog">
		<div class="modal-dialog">
			<!-- Modal content-->
			<div class="modal-content">
				<div class="modal-header">
					<button type="button" class="close" data-dismiss="modal">×</button>
					<h4 class="modal-title">Tambah Data Berita</h4>
				</div>
				<div class="modal-body">
					<div id="pesan_add"></div>
					<?php echo form_open_multipart('berita/insert',array('id'=>'berita_add'));?>
					<input type="hidden" name="current_url" value="<?php echo current_url(); ?>">
					<span>Lampiran</span>
					<div class="panel panel-default">
						<!--UPLOAD IMAGE PREVIEW-->
						<div class="panel-body text-center">
							<img class="img-responsive" src="<?php echo base_url() ?>property/default_attachment.png" alt="Attachment" id="image-preview" style="height:250px; widht:100%;">
						</div>
						<!--UPLOAD IMAGE FILE PICKER-->
						<div class="panel-footer">
							<input type="file" id="attachment" name="attachment" onchange="previewImage();" class="input" />
						</div>
					</div>
					<!--JUDUL BERITA-->
					<div class="input-group">
						<span class="input-group-addon" id="basic-addon1">Judul</span>
						<input type="text" class="form-control" placeholder="Judul Berita" aria-describedby="basic-addon1" style="text-transform:uppercase" name="judul" id="judul">
					</div>
					<br>
					<!--ISI BERITA-->
					<div class="input-group">
						<span class="input-group-addon" id="basic-addon1">Isi</span>
						<input type="text" class="form-control" placeholder="Isi Berita" aria-describedby="basic-addon1" style="text-transform:uppercase" name="isi" id="isi">
					</div>
					<br>

					<button type="submit" class="btn btn-success pull-right">Simpan</button>
					<br>
					<br>
					<?php echo form_close(); ?>
				</div>
			</div>
		</div>
	</div>

//**JS**//

 $('#berita_add').on('submit', function (e) {
		e.preventDefault();
		$form = $(this);
		curr_url = $('[name=current_url]').val();
		formdata = $(this).serialize();
            console.log(formdata);
		$.post(
				$form.attr('action'),
				formdata + '&csrf_qwerty=' + csrf_token,
				function (r) {
					res = $.parseJSON(r);
					if (res.kode == 0) {
						$('#pesan_add').html(res.pesansaya);
					} else {
						window.location = curr_url + '?msg=' + res.kode;
					}
				});
		}

	);

//**Controller**//

 public function validasi_input(){
		if(empty($_FILES['attachment']['name'])){
			$this->form_validation->set_rules('attachment', 'Lampiran','required|trim|xss_clean');
		}
      	$this->form_validation->set_rules('judul', 'Judul','trim|required|min_length[1]|max_length[50]');
		$this->form_validation->set_rules('isi', 'Isi','trim|required|min_length[1]|max_length[100]');
    }

	public function berita_upload_config(){
		$config['upload_path'] = 'uploads/berita';
		$config['allowed_types'] = 'jpg|jpeg|png|gif|bmp';
        $config['encrypt_name'] = TRUE;
		$config['max_size'] = '48000';
		$config['max_height'] = '5000';
		$config['max_width'] = '5000';
		$config['file_name'] = 'berita-'.time();
		$this->load->library('upload', $config);
		$this->upload->initialize($config);
	}

	public function insert(){
		$kode = $this->buat_kode();
		$now = date('Y-m-d H:i:s');
		$user_id = '111111';
		$this->validasi_input();
		if(count($_POST)!=0){
			if($this->form_validation->run()==true){
				$kode_ada = $this->m_berita->periksa_kode($kode);
				if($kode_ada!=0){
					$kode = $this->buat_kode();
				}else{
					$this->berita_upload_config();
					$att_berita = 'attachment';
					if($this->upload->do_upload($att_berita)){
						$file = $this->upload->data();
						$nama_file = $file['file_name'];
						$uk_file = $file['file_size'];
						if(!$uk_file<500){
							$this->resize_config($nama_file);
						}
						$data['id'] 		= intval($kode);
						$data['judul'] 		= $this->input->post('judul');
						$data['isi'] 		= $this->input->post('isi');
						$data['gambar'] 	= $nama_file;
						$data['created_at'] = $now;
						$data['created_by'] = $user_id;
						$data['updated_at'] = $now;
						$data['updated_by'] = $user_id;
						$simpan=$this->m_berita->insert($data);
						if($simpan){
							echo json_encode(array('kode'=>1));
						}else{
							echo json_encode(array('kode'=>2));
						}
					}else{
						$error = array('error' => $this->upload->display_errors());
						echo $error;
					}
				}
			}else{
				echo json_encode(array('kode'=>0,'pesansaya'=>validation_errors()));
			}
		}else{
			echo json_encode(array('kode'=>0,'pesansaya'=>validation_errors()));
		}
	}

Barangkali agan-agan bisa bantu dan kasih tau dimana kesalahan coding saya. Terima kasih