Attributes does not save

[spidercraft]

Im trying to write a simple string as a attribute on a model, but for some reasons after saving my .rbxm file it does not appear, i dont know if im doin anything wrong, but im getting crazy with this lol

RobloxFile file = RobloxFile.Open(rbxmPath);
Model testModel = file.FindFirstChildWhichIsA();
testModel.SetAttribute("cac", "test");
MessageBox.Show(testModel.Name);

file.Save(rbxmPath);