From Xojo Documentation
Allows you to tell whether a given file has its file extension hidden or visible, or toggles that status.
Note that when you change this setting, the operating system's Desktop Manager or subsequent Navigation dialogs may not reflect the change right away. When you create a new file under macOS, the "hide extension" flag is set according to the current value of this property. On Windows, hiding the file extension is a global property, so if it is true for one FolderItem on the user's computer it is true for all of them.
This code checks the ExtensionVisible flag for a file.