mac qt app in the dock does not respond to file drop

Hi, I want to drag a file and let it drop on my qt app icon in the dock (Mac OS X 10.6.8, Qt 4.7.3). On Windows this is working but on mac the qt app does not respond to the drop attempt although I followed the qt drag-and-drop documentation and implemented the functions <dragEnterEvent>, <dragMoveEvent>, <dropEvent> and <setAcceptDrops(true)>. I also use the <QEvent::FileOpen:> event. If I double click on the appropriate file my app opens correctly. What may be wrong? Thanks for your help. Peter