The search plugin does not do any displaying, it only pushes the items for display on the results page. The rest is up to the results page. If the results page is not capable of displaying attachments (or simply removes them from the list), there is unfortuantely nothing we can do about it.
I tried to forcefully change the post type from “attachment” to “page” and “post” when the results loop is sent to the results page – but that did not help either.