The app is missing a listbox. You can have a button to select a folder, like with uigetdir(), but then you should call dir() to load all the files in the folder into a listbox so the user can easily click on one or more files to process them. Don't force the user to browse for a file every time they want to process a new one when they can simply see the one they want and click on it in a listbox. That's just basic user friendly UI design.
Other than that I can't say much since I don't know what your app really does and what the "backend" app does.