Disney really doesn't care what food or drink you bring in unless its alcoholic or in a glass container.
They definitely don't want you to bring in a george foreman grill, some steaks, cans of corn, and a big ol' apple pie. A couple of sandwiches here and there, bottled water, etc... they're all fine.
Think about it this way. What happens when you decide to eat at a monorail resort, have leftovers that you boxed up and you went back to the park. Are they going to stop you? Technically you bought the food on Disney property, and you never left Disney property.
Anyway, I don't pack anything with me except a bottle of water. Why? Because Disney has way too much good food offerings. I can eat PB&J any day at home, but I can't get a turkey leg at home, or Casey's hotdog, or a nice big Mickey ice cream bar at home! One of the pleasures of going to Disney is to eat their food!