When dragging items onto a mail window, if you don’t get them right into an item slot you’re asked if you want to destroy the items. A drag and drop anywhere onto the window should put the item into the nearest (Or first, whatever) open slot in the mail message.
(Had a friend accidentally destroy some items because of clicking, “Yes I want to mail it” since “Do you want to destroy the item you just dragged onto your mail message?” doesn’t make sense)