Upload file using AJAX with CodeIgniter

I have tried many examples. It did succeed ever, but sometimes it didn't load. This is my code:

<div class="fallback">
    <form method="POST" id="quiz_file" action="<?php echo site_url('home/upload_quiz/' . $kelas);?>" enctype="multipart/form-data">
        <input name="filequiz" id="filequiz" type="file" />
    </form>
</div>

      

        
        
        
      

    

This is the ajax code:

$('#submitquiz').on('click', function(event) {
                var inputFile = $('input[name=filequiz]');
                var uploadURI = $('#quiz_file').attr('action');
                document.getElementById('submitquiz').disabled = true;
                var fileToUpload = inputFile[0].files[0];
                // make sure there is file to upload
                if (fileToUpload != 'undefined') 
                {
                    // provide the form data
                    // that would be sent to sever through ajax
                    var formData = new FormData();
                    formData.append("file", fileToUpload);
                    console.log("Process");
                    // now upload the file using $.ajax
                    $.ajax({
                        url: uploadURI,
                        type: 'post',
                        data: formData,
                        processData: false,
                        contentType: false,
                        beforeSend: function ( xhr ) {
                           console.log("progress");
                           console.log(xhr);
                           console.log(formData);
                        },
                        success: function(data) 
                        {
                           console.log("finish");
                           console.log(data);
                        }

      

        
        
        
      

    

This is my controller:

function upload_quiz($kelas) 
    {
        $temp = explode(".", $_FILES["filequiz"]["name"]);
        $extension = end($temp);
        $new_name = time() . "." . $extension; 
        $config['upload_path']          = './assets/images/quiz_images/';
        $config['allowed_types']        = 'gif|jpg|png|jpeg';
        $config['max_size']             = 2000;
        $config['file_name']            = $new_name;
        $config['max_width']            = 1024;
        $config['max_height']           = 768;
        $this->load->library('upload', $config);

        $session_data = $this->session->userdata('logged_in');
        $data['user_name'] = $session_data['user_name'];
        $data['kelas'] = $kelas;
        if ( ! $this->upload->do_upload('filequiz'))
        {
            $data['error'] = array('error' => $this->upload->display_errors());
            $this->load->view('course', $data);
        }
        else
        {
            $data['status'] = array('upload_data' => $this->upload->data());
            $quiz_name = $this->input->post('quiz_name');
            $this->load->model('Model');
            if($this->Model->input_quiz($quiz_name,$new_name,$kelas) == TRUE)
            {
                $this->load->view('course', $data);
            }
            elseif($this->Model->input_quiz($quiz_name,$new_name,$kelas) == FALSE)
            {
                $this->load->view('course', $data);
            }
        } 
    }

      

        
        
        
      

    

After I submit with the button, the AJAX response when I use console.log (data). And it says that the first line in my controller function ($ temp) is UNDIFEND INDEX: filequiz.

What might be missing?