Mastiffs will truly eat nearly anything. I keep him away from most fruit, as grapes (and raisins) are poisonous to dogs. He eats a bite or two of grass in the yard.
But when he is fed, twice a day, it's meat.
When they are puppies, they will eat gravel/small rocks, so you have to watch out for that.
Honestly, this was a joke, at least it started out that way. When did we get so serious about the diet of dogs? As I said, he likes cheap burgers. Once a month or so, he gets them for treats.
Ours also eat carrots, and the diabetic dog has survived 5 years now on lean meat, brocolli, cabbage, (cooked), baked yams, and a few other items. We bake them chicken regularly and feed a high quality dog food.