Codeigniter upload image jquery $_FILES Empty

Question

I make upload image with codeigniter, mysql and jquery, but i get some problem.

This is my code. View

<form id="formupload" method="POST" enctype="multipart/form-data">
<div class="row">
    <div class="col-md-3"><input type="file" name="gambar" class="form-control" multiple accept='image/*' required=""></div>
    <div class="col-md-3"><input type="text" name="title" class="form-control" placeholder="Title (default is blank)"></div>
    <div class="col-md-3"><input type="text" name="description" class="form-control" placeholder="Description (default is blank)"></div>
    <div class="col-md-3"><button type="button" onclick="simpan()" class="btn btn-info btn-fill btn-wd"><i class="fa fa-save"></i> Save</button></div>
</div>

Jquery

  function simpan(){
  $.ajax({
    url : "<?php echo base_url('ruangadmin/slideshow_simpan') ?>",
    type: "POST",
    data: $('#form').serialize(),
    dataType: "multipart/form-data",
    success: function(data){
      if(data.status){
        reload();
        swal('Success', 'Images uploaded', 'success');
      }
    },
    error: function (jqXHR, textStatus, errorThrown){
      swal('Faild', 'try again!', 'error');
    }
  });
}

Controller

public function slideshow_simpan(){
    $this->cek_session();
    $gambar = $_FILES['gambar']['name'];
    $title = $this->input->post('title', TRUE);
    $description = $this->input->post('description', TRUE);

    $config['upload_path']   = FCPATH.'/assets/img/slide/';
    $config['allowed_types'] = 'gif|jpg|png';
    $config['max_size']  = '1000';

    $data = array(
        'gambar' => str_replace(" ", "_", $gambar),
        'title' => $title,
        'description' => $description,
    );

    $this->load->library('upload',$config);
    $this->upload->initialize($config);
    $unggah = array(
        'gambar' => $this->upload->do_upload('gambar'),
    );
    $this->modelapp->insertdata('slide', $data);
    echo json_encode(array("status" => TRUE));
}

i get $_FILES empty data, some field was saved into database but filename not save and file not uploaded. how to fix it?

Thanks.