http://www.lovelongears.com/lionpix.html
The photos are supposed to be of a mule in New Mexico on a lion hunt (yes it is legal in some states). The mule is attacking the lion which we were told initially had already been shot by hunters.
Yes, a mule will attack a predator in this fashion. This is the same way a donkey would attack a dog or coyote, which is what makes (some) donkeys good guard animals. The photos are very graphic, but do show exactly the mode of attack a mule or donkey would use.