Index attachment titles for parent

relevanssi-attachment-titles.php

<?php

// Add this function to your theme functions.php and reindex.
add_filter( 'relevanssi_content_to_index', 'rlv_index_attachment_names', 10, 2 );
function rlv_index_attachment_names( $content, $post_id ) {
	$attachments = get_children(
		array(
			'post_parent' => $post_id,
			'post_type'   => 'attachment',
		)
	);
	foreach ( $attachments as $attachment ) {
		$content .= ' ' . $attachment->post_title;
	}
	return $content;
}