Very curious what exactly "file labor" means... I find it hard to believe that your right forearm is really "noticeable" bigger than your left..... but, learn something new everyday haha. I agree with gymdandee... do some isolation exercises for your forearms and concentrate on higher reps (or weight) with your left forearm to build it up... if "file labor" is what got it there could do that too... and just focus on using your left instead of your right arm
I'm assuming you're right handed if that's the case then do strength training of your left arm.