I left my Chrome downloading a huge file and when I got back after hours, the screen was still shining bright. I think that downloading a file shouldn't prevent the screen from turning off as the screen does not need to stay on. Chrome does announce "downloading a file" but I do not know if that is parseable (if it sets like an enum or somesuch). If it is however, I think it should still turn screen off but _not_ suspend the session while download is in progress; there's no need for screen to stay on during downloads.
Unfortunately the Freedesktop inhibition interface, in contrast to Solid's, only knows everything on or off, so we can either have the screen turn off during a video, or not during a download. The reason stated is just a string, so nothing we could parse. Sorry.