It is generally not good critical form to lift a film’s publicity materials when writing about it, but the official logline for Quentin Dupieux’s “Mandibles” is such a masterpiece of the form that it merits quoting, and admiring, in full: “When simple-minded friends Jean-Gab and Manu find a giant fly trapped in the boot of a car, they decide to train it in the hope of making a ton of cash.” As well as a crisp precis of what the film is about — and let it be stressed that this 77-minute prank of a film about no more than that — it’s an ideal litmus test for its potential audience. If you merely think that sounds like the dumbest thing ever, walk on by. If you think that sounds like the dumbest thing ever and you absolutely have to see it, you won’t be remotely disappointed: “Mandibles” is as brazenly and riotously stupid as it sounds, but with a chill, dopey sweetness that makes it stick.