Source

File: wp-admin/includes/file.php

function get_file_description( $file ) {
	global $wp_file_descriptions, $allowed_files;

	$dirname   = pathinfo( $file, PATHINFO_DIRNAME );
	$file_path = $allowed_files[ $file ];

	if ( isset( $wp_file_descriptions[ basename( $file ) ] ) && '.' === $dirname ) {
		return $wp_file_descriptions[ basename( $file ) ];
	} elseif ( file_exists( $file_path ) && is_file( $file_path ) ) {
		$template_data = implode( '', file( $file_path ) );

		if ( preg_match( '|Template Name:(.*)$|mi', $template_data, $name ) ) {
			/* translators: %s: Template name. */
			return sprintf( __( '%s Page Template' ), _cleanup_header_comment( $name[1] ) );
		}
	}

	return trim( basename( $file ) );
}