Ajax handler for saving backward compatible attachment attributes.

function wp_ajax_save_attachment_compat() {
	if ( ! isset( $_REQUEST['id'] ) ) {
		wp_send_json_error();
	}

	$id = absint( $_REQUEST['id'] );
	if ( ! $id ) {
		wp_send_json_error();
	}

	if ( empty( $_REQUEST['attachments'] ) || empty( $_REQUEST['attachments'][ $id ] ) ) {
		wp_send_json_error();
	}

	$attachment_data = $_REQUEST['attachments'][ $id ];

	check_ajax_referer( 'update-post_' . $id, 'nonce' );

	if ( ! current_user_can( 'edit_post', $id ) ) {
		wp_send_json_error();
	}

	$post = get_post( $id, ARRAY_A );

	if ( 'attachment' !== $post['post_type'] ) {
		wp_send_json_error();
	}

	/** This filter is documented in wp-admin/includes/media.php */
	$post = apply_filters( 'attachment_fields_to_save', $post, $attachment_data );

	if ( isset( $post['errors'] ) ) {
		$errors = $post['errors']; // @todo return me and display me!
		unset( $post['errors'] );
	}

	wp_update_post( $post );

	foreach ( get_attachment_taxonomies( $post ) as $taxonomy ) {
		if ( isset( $attachment_data[ $taxonomy ] ) ) {
			wp_set_object_terms( $id, array_map( 'trim', preg_split( '/,+/', $attachment_data[ $taxonomy ] ) ), $taxonomy, false );
		}
	}

	$attachment = wp_prepare_attachment_for_js( $id );

	if ( ! $attachment ) {
		wp_send_json_error();
	}

	wp_send_json_success( $attachment );
}