I'm having a problem uploading images using "Add image URLs" in the latest Firefox release. I tried it on your demo as well and still nothing is happing, please advise.
First I applied your hot fix the two files and that did not work- so I re-downloaded the files and uploaded them and it still does not work, also I sometimes would get that message.
Some files couldn't be added
npoiuu...com/.../wallpapers-10.jpg
I cashed my browser, tried it in safe mode flushed DNS tried it on two different computers and I'm still getting the same results. Maybe you can give it a try.