Prompts the user to select an attachment to add to this edit.

def pickAttachment(self):
        """
        Prompts the user to select an attachment to add to this edit.
        """
        filename = QFileDialog.getOpenFileName(self.window(),
                                               'Select Attachment',
                                               '',
                                               'All Files (*.*)')
        
        if type(filename) == tuple:
            filename = nativestring(filename[0])
        
        filename = nativestring(filename)
        if filename:
            self.addAttachment(os.path.basename(filename), filename)