WordPress - Short page template body classes

short-page-template-body-classes.php

function short_page_template_body_classes( $classes ){
	if( is_page() ){
		$template = get_page_template_slug(); // returns an empty string if it's loading the default template (page.php)
		if( $template === '' ){
			$classes[] = 'default-page';
		} else {
			$classes[] = sanitize_html_class( str_replace( '.php', '', $template ) );
		}
	}
	return $classes;
}
add_filter( 'body_class', 'short_page_template_body_classes' );