In the FileChooser constructor we can specify the filePatternsAllowed parameter.
Using an OS native window, in a save file dialog these file type options are listed in a combo and the user can choose e.g. jpg or png.
If he types a filename and validates the dialog, the returned File object does not contain the extension (except if it was explicitily written there by the user).
How am I supposed to find out that he actually wants to save a jpg or a png?