The TypeError: Illegal invocation occurs when adding a $watch with objectEquality set to true on a model which contains a file(s) property used by ngf-drop.
Also as reported here.
How can I $watch the model without getting this error?
Solved without having to use a separate scope variable for the files.
Instead of $watching the model, I $watched a function that returns Json.stringify(model) -- with objectEquality true.
It works, and does not seem to harm perfomance in a user-visible manner.