Ghost image in Add Media modal attachment browser

I upgraded to the 3.6 beta from a 3.5.1 base. I then edited the “Hello world!” sample post and clicked “Add Media”. On the “Insert Media” modal window, “Media Library” tab the first item in the media items browser is a pale gray square with a small black icon. Selecting this item displays Attachment Details with blank Title, Caption and Description fields. The “div class=filename” node in the HTML source is empty.

All the other items in the browser are correct. The “ghost” item is an additional item, not a corruption of an existing item.