在Codeigniter 3中上载的情况下如何显示不允许的文件类型错误消息？

我正在使用Codeigniter 3.1.8和Bootstrap 4开发一个基本的博客应用程序。

这些帖子当然有主要图片。有一个默认的图像，如果没有图像是由用户上传的，但是，如果图像被上传，也有只有3个允许类型：JPG，JPEG和PNG。

想要警告用户，以防他/他尝试上传其他文件格式，我在Posts控制器中执行了此操作：

// Upload image
$config['upload_path'] = './assets/img/posts';
$config['allowed_types'] = 'jpg|jpeg|png';
$config['max_size'] = '2048';

$this->load->library('upload', $config);

if(!$this->upload->do_upload()){

    $data['uerrors'] = $this->upload->display_errors();

    if ($data['uerrors']) {
        $this->load->view('partials/header', $data);
        $this->load->view('dashboard/create-post');
        $this->load->view('partials/footer');
    } else {
        $post_image = 'default.jpg';
    }

} else {
    $data = array('upload_data' => $this->upload->data());
    $post_image = $_FILES['userfile']['name'];
}

我认为：

<?php foreach ($uerrors as $uerror): ?>
  <span><?php echo $uerror; ?></span>
<?php endforeach; ?>

但是，我得到了一个未定义的变量：uerrors错误。

这是整个create()方法：

public function create() {

    // Only logged in users can create posts
    if (!$this->session->userdata('is_logged_in')) {
        redirect('login');
    }

    $data = $this->get_data();
    $data['tagline'] = "Add New Post";

    if ($data['categories']) {
        foreach ($data['categories'] as &$category) {
            $category->posts_count = $this->Posts_model->count_posts_in_category($category->id);
        }
    }

    $this->form_validation->set_rules('title', 'Title', 'required');
    $this->form_validation->set_rules('desc', 'Short description', 'required');
    $this->form_validation->set_rules('body', 'Body', 'required');
    $this->form_validation->set_error_delimiters('<p class="error-message">', '</p>');

    if($this->form_validation->run() === FALSE){
        $this->load->view('partials/header', $data);
        $this->load->view('dashboard/create-post');
        $this->load->view('partials/footer');
    } else {
        // Create slug (from title)
        $slug = url_title(convert_accented_characters($this->input->post('title')), 'dash', TRUE);
        $slugcount = $this->Posts_model->slug_count($slug, null);
        if ($slugcount > 0) {
            $slug = $slug."-".$slugcount;
        }

        // Upload image
        $config['upload_path'] = './assets/img/posts';
        $config['allowed_types'] = 'jpg|jpeg|png';
        $config['max_size'] = '2048';

        $this->load->library('upload', $config);

        if(!$this->upload->do_upload()){

            $data['uerrors'] = $this->upload->display_errors();

            if ($data['uerrors']) {
                $this->load->view('partials/header', $data);
                $this->load->view('dashboard/create-post');
                $this->load->view('partials/footer');
            } else {
                $post_image = 'default.jpg';
            }

        } else {
            $data = array('upload_data' => $this->upload->data());
            $post_image = $_FILES['userfile']['name'];
        }

        $this->Posts_model->create_post($post_image, $slug);
        $this->session->set_flashdata('post_created', 'Your post has been created');
        redirect('/');
    }
} 

我的错误在哪里？