The Drag-n-Drop feature to send a file is better than selecting that file using the browse option. But the drap-n-drop has the disadvantage of switching between windows (more problem if we have many windows opened) with the file being held by the mouse. Also if we have multiple chat boxes opened, then we have to move the mouse over that to select the chat box to which we would drop the file.
Instead of that why can't we use the right-click->'Send To' option? I would right click on the file to be sent. Then move over the 'Send To' option. This would give me a drop down of the contacts for which the chat boxes are open. Also, I would have the search bar in which I type the names if I want to send to other contacts for whom chat boxes aren't open. Then, I just select the contact and he/she gets the notification to accept/reject the file.