When an operator is attached to a file select dialog, it updates the directory, filename and filepath properties of the operator (if they exist) as the user navigates through the dialog.

As an operator author, I want to be notified when these properties are changed so that I can provide contextual options based on the contents of the selected file (for example, providing a list of elements available to import from the file to pick from).

This patch does just that. I've attached a .blend file to demonstrate the functionality. You will need to enable developer extras, run the script in the text editor and then press F3 and search for "filepath". The operator will appear in the list. Select it, then navigate around your file system. The name of the filepath and the size of the file will be updated as you select different files.

Here is a demo of a more real-life scenario, applied to an add-on I maintain where it imports animations from a file that contains an arbitrary amount of animations within it (real-world files have upwards of 2000 animations). The video demo shows it populating a list of animations from the selected file.