Yeah, I had forgotten about that change. But I didn't know it had anything to do with gifs. In fact, I'm already using that component (which was the reason that I updated to 5.1 to begin with). I've told the developer and he will take a look at it.
And indeed, changing it back fixed the problem. The only thing is that when you edit a post, the change isn't shown until you refresh the page. In 5.0 you didn't have to if I remember it correctly.