Looking at the small violent nature.of the injury, I have to think a shaped charge of some sort. Otherwise, the right leg would have seen some injury. That said, commercial fireworks that you can buy at the fireworks stand, include shaped charges. Roman candles and other similar fireworks are shaped charges. The question is, what was used to detonate the charge? Not the normal fireworks.
Directional charge, yes. Shaped charge, no. Originally designed as an anti-tank weapon, but more commonly used as a mining charge before OIF made them famous, a shaped charge utilizes a void cut into the explosive. Inside the hollow area of that cone, you will find a liner (typically metal, but glass can work). The main charge is evenly packed around the outside of that cone. When it detonates, assuming the HE is packed properly and the initiator is in the right place, the Munroe Effect occurs. In layman's terms, the cone is evenly compressed, and the liner is forced forward, forming a jet.
It is possible to channel the direction of the blast wave. For example, using IV bags or water bottles on one side of a main charge will tamp the blast on one side, and allow the blast to continue on in the other direction. Another example is when a breaching team utilizes explosive tools in a hallway. Each time the blast wave reflects off of a wall, it grows stronger. That's why a breaching team would be safer staging closer to the charge in that instance.
That said, based on the one photo I've seen, the damage to his foot is indicative of blast injury, but not a shaped charge injury. It would've been (at least somewhat) cleanly severed if it were in line with the jet, as opposed to blown off. While I have ideas for what the main charge could've been, I can't make any guesses without a little more information (color of smoke, etc). It wasn't large, and it could've been anything from TATP to Armstrong's mixture. From what I've gathered, the main charge was placed beneath the rock that he stepped on. The earth below would serve to reflect the blast wave back up towards the more mobile rock. The rock itself would then deflect the blast wave somewhat, which could be why his foot was only mangled all to hell, as opposed to becoming pink mist. This is just from a small charge.
While it's better to not have been blown up to start with, he got lucky. As far as who placed it, I'm going to withhold my speculation for the time being. If it turns out that the main charge was actually a primary charge, then the emplacer was also lucky (not to mention stupid).