From Xojo Documentation
The control is about to be displayed. Use this event to initialize/setup a control.
This event is called after the Constructor.
Be warned that setting control property values in the Constructor instead of the Opening event may result in those property values being overwritten by what is set in the Inspector. For best results, use the Opening event for setting the initial values of control properties rather than the control Constructor.
If the control is supposed to handle drag and drop, you need to tell it which type of item it needs to be able to handle. The following example informs the control that pictures and files can be dropped on it. The type of the file it needs to support is specified via the File Types Editor.