My brackets were broken in a couple of spots, which typically wouldn't happen to mild steel. Spring steel will bend to some extent, but exhibit quite a bit of rebound, and if bent too much, will eventually fracture.
I have the pieces of mine in my scrap pile, and can try bending them, but I'll lay money on it that they're spring steel. A high carbon steel will also exhibit a different spark pattern than mild steel if you use a grinder on them.
Here's a couple of links on spark testing.
Spark Testing 1 Spark Testing 2
