WP: Insert a file from form into wordpress media library

//**************************************
// Attachment Insert Form
//**************************************
function insert_attachment_form($postID) {
?>
    <form id="file-form" name="file-form" method="POST" action="" enctype="multipart/form-data" >
        <input type="file" id="async-upload" name="async-upload" />
        <input type="hidden" name="postID" value="<?php echo $postID; ?>" />
        <?php wp_nonce_field('client-file-upload', 'client-file-upload'); ?>
        <input type="submit" value="Upload" id="submit" name="submit" />
    </form>
<?php }

//**************************************
// Process Attachment Form
//**************************************
function process_attachment() {
    // verify this came from the our screen and with proper authorization,
    // because save_post can be triggered at other times
    if ( !wp_verify_nonce( $_POST['client-file-upload'], 'client-file-upload') ) {
        return $post->ID;
    }

    // Is the user allowed to edit the post or page?
    if ( !current_user_can( 'publish_posts', $post->ID ))
        return $post->ID;

    if( 'POST' == $_SERVER['REQUEST_METHOD'] && !empty( $_FILES )) {
        require_once(ABSPATH . 'wp-admin/includes/admin.php');
        $id = media_handle_upload('async-upload', $_POST['postID']);
        unset($_FILES);
    }
}

add_action('init', 'process_attachment');