Adding drag-and-drop support for editable files in my NBP-based app

I'd like users of my NBP-based application to be able to drag a file from the file manager (e.g. Windows Explorer) to my application, and have it opened with the associated editor.

I've got a DataObject defined and registered (via DataObject.Registration, MIMEResolver.ExtensionRegistration, and several ActionReferences for open/cut/copy).  I set breakpoints in DefaultExternalDropHandler, which appears to be responsible for this functionality, but these breakpoints are never reached.  When I drag any file over, I just see the "no" cursor over all parts of the application.

I also tried capturing FINE messages from all org.netbeans and org.openide packages for my app, and I see nothing logged in response to dragging files over the application -- and nothing at all logged mentioning "external", "drag", or "drop" during the whole application lifetime.

I can't figure out what else I am missing -- any help would be appreciated.