It appears that the markup created with a drag/drop or copy/paste is different from the markup created by the file upload tool.
<img src="https://aws1.discourse-cdn.com/mcneel/uploads/default/original/3X/8/0/80a0422d81dce17ec40f0938a43ae9a77e8a6d78.png" width="690" height="379">
I’ll let the folks at Discourse know you prefer the latter.
I agree about the button. We hacked in the filepicker.io stuff to get generic uploads working; the Discourse team is working to integrate it properly into their system.