Also I learned something valuable yesterday. If you type .mp4 of a gif link instead of .gif, it converts them to mp4 and actually downloads them. After using Tumblr for 4 years! It's true when they say you learn something new everday.
Late answer from Bob 30/1/20 I didn't know that. I always use google downloader to get gifs from twitter and that also converts them automatically to mp4.