What do you get when you attach 1 flamethrowers to a mini train wagon? A mini flamethrower wagon of course! Based on the mod flamethrower-wagon by fgardt.
You are missing the name and description for the turret.
under [entity-name] you need to add:
mini-flamethrower-wagon-turret=Mini Flamethrower Wagon Turret
under [entity-description] you need to add:
mini-flamethrower-wagon-turret=A flamethrower turret attached to a fluid wagon.