Coolant is flammable, oil is flammable, power steering and transmission fluid are flammable (my car had a PS hose recall for this reason), oil is flammable, and gas is VERY flammable. A little bit of propane shouldn’t be too scary.
That being said, you should keep one of these in your car anyways.
If you use a dry powder extinguisher, make sure to tip it back and forth every once in a while to prevent the powder from clumping on the bottom. You should be able to hear and feel it moving.