If original, the heads should take a flat screwdriver. This creates a problem since you can't apply much torque to those type of screw heads. It really isn't necessary for these bolts to have that kind of head since the thick, spongy padding under the floor mat will easily hide a hex head even with a washer under it.
Grind the heads off with an angle grinder, then take the inner fender to the bench where you can more easily spray them with Kroil or similar, or apply heat to the captured nut, and so that you can get vice grips on the exposed threads (visible in your pic) and easily unscrew them.
Carl
