We've gotten so used to drag-and-drop working everywhere that when an app refuses to accept an image or document that's clinging desperately to your cursor, it comes as a surprise. The "drag" part of the operation is usually more restricted, except in the case of Google Chrome, where even the download bar is getting in on the action.
It's no secret that the major browsers support drag-and-drop from your computer's file browser, but Chrome wants to save you even more time by allowing you to click-and-hold on a downloaded file in the download bar and drop it wherever you want.
This could be a time saver if you've downloaded some images from cloud storage and want to upload them to Facebook or a hosting service without ever having to leave your browser.
However, once you need to upload a large amount of files, you're better of using Explorer or Finder and taking advantage mass selection. Otherwise, this tip should help streamline simpler download/upload tasks.