I recently came across this image which seems like a normal GIF:
Even imgur stores it as a GIF when I uploaded it here.
But when I saw the link, it was a JPEG image, unlike a GIF than what I had expected!
Here's the link: https://fbcdn-sphotos-a.akamaihd.net/hphotos-ak-ash3/559307_325955130827518_630404691_n.jpg
This is actually a .gif - look at the headers of the HTTP response.
The MIME type is image/gif
So it doesn't matter what the URL says at the end - it's the MIME type that determines how your browser interprets it.
A lot of sites use this (I guess to throw off harvesters?) imgur.com often reports the wrong extension, for example. Try going to i.imgur.com/yourpic.txt - it should still show up as a proper gif.
来源：https://stackoverflow.com/questions/11529941/file-with-jpg-extension-acts-like-a-gif-with-a-cat-shooting-a-gun-with-heavy-kic