Hi,
This might be much more complicated than I thought. May I ask which plugin do you use exactly for these attachments? I will have to look into their API, as there is no universal solution to get any attachments related to a certain post, unless it is within the media library. Even worse, if the items are not custom post types.
Indexing Media attachments along with their parent posts could be doable, but when a file is outside the Media library, it becomes a nightmare.
Well, the indexing process is a bit complicated, there are multiple files involved. The main file that controls the indexing is wp-content/plugins/ajax-search-pro/includes/classes/etc/indextable.class.php
However I honestly does not recommend modifying that, it could be extremely time consuming.