This seems to be an animated GIF with headers good enough for transferring in JPEG format. This is not ideal, most of my system file viewers are suffocating, but this is apparently enough for Facebook. Your browser is written with more fault tolerance and automatically detects that it is truly a GIF and displays the animation.
This, of course, is an exploit. If you sent the image to Facebook or tried to repeat the trick in another movie, you may be blocked.
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