From Xojo Documentation
| This property is only available on the macOS platform.
For cross-platform development, use ＃If...＃Endif with the Target... specifiers to make sure you will not attempt to use this property on an incompatible platform.
Specifies which file types the application will accept via drag and drop.
Applies only to the initial or “blessed” instance based on the Application class and is for Macintosh only. Use the File Type Set Editor to enter the file types and in the App object's Properties pane, click the "..." button to open a dialog box in which you can edit this list.