Site admin > Plugins > Activity modules > File > Require activity description.
As for why you can't add instructions to assignments via drag + drop. Click on 'reply' to this message. You will see 2 boxes - one to type in, one to upload files (via the 'add' button or drag + drop). If you do the same on a Moodle 1.9 site you will see 2 boxes - one to type in, one to upload files (via the 'add' button, no drag + drop option).
Now do the same on an assignment settings page. On both a 1.9 site and a 2.5 site you will see one box to type in a description and no box to upload a description as a file.
The drag and drop feature updated all existing file-upload 'boxes', so that they could accept files being dragged + dropped. This feature did not automatically add file-upload boxes where none existed already (as is the case on the assignment settings page), but just improved the existing file-upload boxes (such as here on this forum).
You could certainly make the case that it would be helpful to have a file-upload option on the assignment settings page, but, unless someone specifically decides to add such a new feature it won't just happen. This is why I suggest going to tracker.moodle.org and putting in a request for this new feature (after searching to see if anyone has already requested it).