Downloads: How to make files appear in parent categories?

[MMXII]

Yes, it is an old topic, but sadly I have to bring it up again, since the Downloads app still (!) has this weird behavior to make files only appear in subcategories, but not in parent categories. I hate that. No one wants a "There is nothing here yet" block on a parent category when there are lots of subcategories full of interesting files. Why does it not work like a filter, so that a parent category displays all the content of its subcategories, and by selecting a subcategory you only get to see its content? And why can't we select more than one category per file?

Anyways, here is my question: Is there an easy way to make files appear in parent categories?

I know there has once been a plugin for this, but unfortunately I was not able to find it via search. Also it is probably outdated by now, because I remember this being an issue in 4.1 times or even earlier.