Did you try with the original file or a copy to *.bmp. ?
I thought it might be a D and so did a flood fill in the middle of it to see what would happen and it worked. I then did an MD5 of the file but the answer was in lower case and so did not work when I pasted it in!
In the original file, with your method, you get an "U" instead of "D" when you floodfill with clicking one pixel at top.
BTW: Why did you not solve the challenge? Did you have had the same File / MD5 troubles like me?