Has wayyy more to do with roughage. Brownies would be pretty easy actually... they're considered "GI soft" (anything you can cut with the side of your fork) so as long as they aren't whole grain or anything (and no nuts, obviously) you could eat them a few days after surgery
The main issue with brownies would be that chocolate and sugar can decrease transit time leading to possibly liquidy output.
The foods that thicken me the most are bagels and peanut butter. I have naturally thin output because my loop ileostomy is pretty high (about
20% of my small intestine is "offline" right now) so I'm always eating thickening foods purposely (marshmallows, potatoes, cheese, pretzels, potato chips) but I have had a couple of incidents where Imodium + bagel or Imodum + too much peanut butter thickened me up too much.
Anyway, it suffices to say that you can eat practically anything with an ileostomy once you're healed up. The *amount* might need to be restricted and you might need to eat certain foods with other foods to move them along (e.g. popcorn with a piece of bread), but in general the dietary restrictions are pretty loose.