Source

File: wp-includes/class-wp-query.php

public function is_attachment( $attachment = '' ) {
		if ( ! $this->is_attachment ) {
			return false;
		}

		if ( empty( $attachment ) ) {
			return true;
		}

		$attachment = array_map( 'strval', (array) $attachment );

		$post_obj = $this->get_queried_object();

		if ( in_array( (string) $post_obj->ID, $attachment, true ) ) {
			return true;
		} elseif ( in_array( $post_obj->post_title, $attachment, true ) ) {
			return true;
		} elseif ( in_array( $post_obj->post_name, $attachment, true ) ) {
			return true;
		}
		return false;
	}