Short answer: it disappears.
Long answer: I had a pic on my local pc I wanted to drag and drop. When I dragged it over the upload box to attach, I could see the large green border effect appear for the file upload box, but after dropping the image into the box, it appeared for a couple seconds and then disappeared. No error message, no symbol, nothing. Since there was no error message, I ran through some quick checks. I double checked that the image was stored locally on my pc (it was), checked the file size wasn't crazy big (~ 2 MB), and checked the image dimensions (huge, like 2000 x 4000 pixels). So I shrunk the dimensions of the image by a lot, tried again, and it worked.
So, while I could figure this out on my own, others may not be able to or not have the patience to bother trying. Adding user friendly error messages will make the site more user friendly.