Hi Celin,
I believe the Elementor posts widget is not capable of that. I recall someone had a similar query about a month ago, and we tried everything to somehow force those items there, but it’s simply not possible, Elementor clevery checks for the post type and dismisses them.
If your theme has a regular results page, that should work with attachments as well. The WordPress core query does not remove the attachments from results – so unless your theme does not remove them, that should work.