So I just had this problem myself and shortly after submitting a support ticket I found a workaround... for me anyway, but I thought I'd share in case it happens to save someone else some time and frustration.
Problem: While trying to upload photos for enhanced verification they seem to upload to 100% but then fails. My files are .jpg and about 3.7MB for my ID and just over 1MB for the selfie (crappier front camera). According to bittrex your files have to be under 8MB however no matter which browser I used or whether I logged out and back in it still wouldn't work.
Solution: Just for the hell of it I decided to try and upload my selfie (1MB) instead of the license (3.7MB) and it worked. So then I selected choose file again and uploaded the right one of my license and it worked. I had the same issue for the picture of the back of my license. Luckily the same workaround worked for that as well. Uploading the selfie had no problem and everything was submitted.
I don't know if this was a problem on my end or their end, but if you have the same problem give it a shot and maybe you'll get lucky.