function wp_delete_attachment( $post_id, $force_delete = false ) {
    ...
    $meta = wp_get_attachment_metadata( $post_id );
    ...
    if ( ! empty($meta['thumb']) ) {
        // Don't delete the thumb if another attachment uses it.
        if (! $wpdb->get_row( $wpdb->prepare( "SELECT meta_id FROM $wpdb->postmeta WHERE meta_key = '_wp_attachment_metadata' AND meta_value LIKE %s AND post_id <> %d", '%' . $wpdb->esc_like( $meta['thumb'] ) . '%', $post_id)) ) {
            $thumbfile = str_replace(basename($file), $meta['thumb'], $file);
            /** This filter is documented in wp-includes/functions.php */
            $thumbfile = apply_filters( 'wp_delete_file', $thumbfile );
            @ unlink( path_join($uploadpath['basedir'], $thumbfile) );
        }
    }
    ...
}

...
switch($action) {
...
    case 'editattachment':
        check_admin_referer('update-post_' . $post_id);
        ...
        // Update the thumbnail filename
        $newmeta = wp_get_attachment_metadata( $post_id, true );
        $newmeta['thumb'] = $_POST['thumb'];
 
        wp_update_attachment_metadata( $post_id, $newmeta );
...

add_filter( 'wp_update_attachment_metadata', 'rips_unlink_tempfix' );
function rips_unlink_tempfix( $data ) {
    if( isset($data['thumb']) ) {
        $data['thumb'] = basename($data['thumb']);
    }
 
    return $data;
}