Cara membuat fungsi upload di ci

function save()
     {

     	$getkodeticket = $this->model_app->getkodeticket();

    	$ticket = $getkodeticket;

     	$id_user = strtoupper(trim($this->input->post('id_user')));
     	$tanggal = $time = date("Y-m-d  H:i:s");

     	$id_sub_kategori = strtoupper(trim($this->input->post('id_sub_kategori')));
		$id_kategori = strtoupper(trim($this->input->post('id_kategori')));
		$id_kondisi = strtoupper(trim($this->input->post('id_kondisi')));
     	$problem_summary = strtoupper(trim($this->input->post('problem_summary')));
     	$problem_detail = strtoupper(trim($this->input->post('problem_detail')));
     	$id_teknisi = strtoupper(trim($this->input->post('id_teknisi')));

     	$data['id_ticket'] = $ticket;
     	$data['reported'] = $id_user;
     	$data['tanggal'] = $tanggal;
     	$data['id_sub_kategori'] = $id_sub_kategori;
     	$data['problem_summary'] = $problem_summary;
     	$data['problem_detail'] = $problem_detail;
     	$data['id_teknisi'] = $id_teknisi;
     	$data['status'] = 1;
     	$data['progress'] = 0;

        //proses upload file

        //         $config['upload_path'] = './assets/files/'; //path folder
        //         $config['allowed_types'] = 'pptx|pdf|doc|docx|ppt|rar|zip'; //type yang dapat diakses bisa anda sesuaikan
        //         $config['remove_spaces'] = TRUE;
        //         $config['overwrite'] = FALSE;
        //         $config['min_size'] = 0;
        //         $config['max_size'] = 10000;

        //  $this->upload->initialize($config);

        $filesize = 0;
        if(is_array($_FILES)){
            if($_FILES['file_attachment']['name']!=""){
                $filesize   = $_FILES['file_attachment']['size'];
                $data['filename_attachment'] = $_FILES['file_attachment']['name'];
                $data['file_attachment'] = file_get_contents($_FILES['file_attachment']['tmp_name']);
                 $fileDestination = './assets/files'.$data;
                    move_uploaded_file($data, $fileDestination);
            }
        }



        // end upload

     	$tracking['id_ticket'] = $ticket;
     	$tracking['tanggal'] = $tanggal;
     	$tracking['status'] = "Created Ticket";
     	$tracking['deskripsi'] = "";
     	$tracking['id_user'] = $id_user;

        if($filesize > 10000000){
            $this->session->set_flashdata("msg", "<div class='alert bg-success' role='alert'>
            <a href='#' class='close' data-dismiss='alert' aria-label='close'>×</a>
            <svg class='glyph stroked empty-message'><use xlink:href='#stroked-empty-message'></use></svg> Ukuran file terlalu besar (max 10Mb).
            </div>");
            redirect('myticket/myticket_list');
        }else{
         	$this->db->trans_start();

         	$this->db->insert('ticket', $data);
         	$this->db->insert('tracking', $tracking);

         	$this->db->trans_complete();

         	if ($this->db->trans_status() === FALSE){
        		$this->session->set_flashdata("msg", "<div class='alert bg-danger' role='alert'>
        	    <a href='#' class='close' data-dismiss='alert' aria-label='close'>×</a>
        	    <svg class='glyph stroked empty-message'><use xlink:href='#stroked-empty-message'></use></svg> Data gagal tersimpan.
        	    </div>");
        		redirect('myticket/myticket_list');
        	}else{


                // email ke admin
                $akses = $this->db->query("select username FROM user WHERE level = 'ADMIN'")->result();
				$username_tiket = $this->db->query("SELECT nama FROM karyawan WHERE nik = '$id_user' ")->row();
				$nama_user_tiket = $username_tiket->nama;

				$row_kategori = $this->db->query("SELECT nama_kategori FROM kategori WHERE id_kategori = '$id_kategori' ")->row();

				$nama_kategori = $row_kategori->nama_kategori;

				$row_sub_kategori = $this->db->query("SELECT nama_sub_kategori FROM sub_kategori WHERE id_sub_kategori = '$id_sub_kategori' ")->row();

				$nama_sub_kategori = $row_sub_kategori->nama_sub_kategori;

				$row_kondisi = $this->db->query("SELECT nama_kondisi FROM kondisi WHERE id_kondisi = '$id_kondisi' ")->row();

				$nama_kondisi = $row_kondisi->nama_kondisi;


                if($akses!=null){
                    foreach($akses as $data){
                        $row_user = $this->db->query("SELECT email FROM user WHERE username = ".$data->username)->row();
                        $to = $row_user->email;


						$data_tiket = array(
							'no_tiket' => $tracking['id_ticket'],
							'user_tiket' => $tracking['id_user'],
							'username_tiket' => $nama_user_tiket,
							'sub_kategori_tiket' => $nama_sub_kategori,
							'kategori_tiket' => $nama_kategori,
							'kondisi_tiket' => $nama_kondisi,
							'subject_masalah' => $problem_summary,
							'deskripsi_masalah' => $problem_detail,
						);

						$body_email = $this->load->view('email/template.php',$data_tiket,TRUE);
						$message = $body_email;
						$subject = $tracking['id_ticket'];
                        $this->load->helper('Email');
                        send($to,$message,$subject);
                    }
                }
        		$this->session->set_flashdata("msg", "<div class='alert bg-success' role='alert'>
        	    <a href='#' class='close' data-dismiss='alert' aria-label='close'>×</a>
        	    <svg class='glyph stroked empty-message'><use xlink:href='#stroked-empty-message'></use></svg> Data tersimpan.
        	    </div>");
        		redirect('myticket/myticket_list');
        	}
        }
    }
}

tolong aku suhu aku bingung cara bikin upload file, masih newbie :(