Now we have Drag&Drop functionality that is quite useful sometimes, but it is only half implemented. It necessary enhance Drop operation to allow make Drop more accurate - in precise target window area.
One of use cases:
1. Open Thunderbird, create new mail place it on the left side of screen.
2. Now you need to Drag&Drop pictures to Attachment, not in body of mail. Focus any application for browsing file system on the right side of screen.
3. Drag few files to Mail, without raising "New Mail" window you did not have ability to put it in attachment placeholder.
FYI, related bugs that almost dead for no reason: