Returns the mime data for dragging for this instance.
        
        :param      items | [<QtGui.QTreeWidgetItem>, ..]

def mimeData(self, items):
        """
        Returns the mime data for dragging for this instance.
        
        :param      items | [<QtGui.QTreeWidgetItem>, ..]
        """
        func = self.dataCollector()
        if func:
            return func(self, items)
        
        data = super(XTreeWidget, self).mimeData(items)
        
        # return defined custom data
        if len(items) == 1:
            try:
                dragdata = items[0].dragData()
            except AttributeError:
                return data
            
            if not data:
                data = QtCore.QMimeData()
            
            urls = []
            for format, value in dragdata.items():
                if format == 'url':
                    urls.append(QtCore.QUrl(value))
                else:
                    data.setData(format, QtCore.QByteArray(value))
            
            data.setUrls(urls)
            return data
        
        return data