Can he play action cards to increase the attack roll?
I always assumed the answer was no. The only possibility that could even possibly apply would be Crossfire. Crossfire can be played prior to making a Fire Attack Roll. With mines, you make a Mine Attack Roll, which "follows the same general rules as for a Fire Attack Roll" (F103.2). I think that means it's still not a Fire Attack Roll, it just follows the same rules, and so Crossfire can't be played. But I'm not 100% sure.