自定义元框上传文件

I have a plugin that adds a meta box with file upload form, the plugin works perfectly, displays the link of the image in the meta box (View Image), load the file, and once published the post, it views in the theme. (in the example I'm using an image file):

<?php
/* Plugin Name: Custom Meta Upload Template*/

add_action('post_edit_form_tag', 'post_edit_form_tag');
function post_edit_form_tag() {
echo ' enctype="multipart/form-data"';
}
function custom_upload() {
add_meta_box( 'meta_upload', __( 'Upload Image', 'meta_upload_domain' ),'meta_upload_callback', 'post' );}

add_action( 'add_meta_boxes', 'custom_upload' );
function meta_upload_callback( $post ) {
global $post;
$custom         = get_post_custom($post->ID);
$download_image01    = get_post_meta($post->ID, '_image01', true);
echo '<p><label for="document_file">Upload document:</label><br />';
echo '<input type="file" name="document_file" id="document_file" /></p>';
echo '</p>';
if(!empty($download_image01) && $download_image01 != '0') {
echo '<p><a href="' . wp_get_attachment_url($download_image01) . '">View Image</a></p>';
   }
}

function meta_upload_save( $post_id ) {

global $post;

if(strtolower($_POST['post_type']) === 'page') {
    if(!current_user_can('edit_page', $post_id)) {
        return $post_id;
    }
}
else {
    if(!current_user_can('edit_post', $post_id)) {
        return $post_id;
    }
}

if(!empty($_FILES['document_file'])) {
    $file   = $_FILES['document_file'];
    $upload = wp_handle_upload($file, array('test_form' => false));
    if(!isset($upload['error']) && isset($upload['file'])) {

        $title      = $file['name'];
        $ext        = strrchr($title, '.');
        $title      = ($ext !== false) ? substr($title, 0, -strlen($ext)) : $title;
        $attachment = array(
            'post_mime_type'    => 'image',
            'post_title'        => addslashes($title),
            'post_content'      => '',
            'post_status'       => 'inherit',
            'post_parent'       => $post->ID
        );

        $attach_key = '_image01';
        $attach_id  = wp_insert_attachment($attachment, $upload['file']);
        $existing_download = (int) get_post_meta($post->ID, $attach_key, true);

        if(is_numeric($existing_download)) {
            wp_delete_attachment($existing_download);
        }

        update_post_meta($post->ID, $attach_key, $attach_id);
       }
    }
 }
 add_action( 'save_post', 'meta_upload_save' );

and he called the file in the theme:

 <?php 
  $download_image01    = get_post_meta($post->ID, '_image01', true);
  if(!empty($download_image01) && $download_image01 != '0') { ?>
        <div class="section-content">
  <img src="<?php echo wp_get_attachment_url($download_image01); ?>" alt=""  />             
        </div>

<?php  }?>

I repeat, the plugin works, but the problem is that I can not place a button or a checkbox that deletes the file when, for example, I want to edit the post, and delete the file, and of course the link "View Image" disappears

Any idea!!

Whenever you want to edit these fields echo the value of the custom fields look like this...

<input type="" value="<img src="<?php echo wp_get_attachment_url($download_image01); ?>" alt=""  />">

When you edit the post it's get the value and show the uploaded image's to you.