From Xojo Documentation
Permits documents of type FileType to be dropped on the control. FileType must be a file type that you defined in via the FileType class or the File Type Sets Editor.
This code in the Opening event of an DesktopCanvas makes it possible for the user to drop either a picture or a file that is a jpeg image. The File Type Sets editor was used to define the “image/jpeg” file type. It is one of the “Common File Types” that is available in the editor.
To restrict file drops to just folders (and not files), you can put this code in the DragEnter event: