A callback function in your case is just a method of a class. You called your class file1 apparently. The Save method doesn't depend on the file1 obj. So can be called by creating a temp file1 object, and then call the Save method of that file1 object.
The only thing you need to worry about is the last input argument (called app in your file). Not sure how app is used inside the Save method. You might need to find the correct app when you call Save.