QtCore.QObject.connect(self.action, QtCore.SIGNAL('triggered()'), self.showDialog)
    def showDialog(self):
        fileName = QtGui.QFileDialog.getOpenFileName(None, "Load Data", "/home/", "xml-files (*.xml)")     
        tree = etree.parse(fileName)
        root = tree.getroot()

# -*- coding: utf-8 -*-
import sys
from PyQt4 import QtCore, QtGui
 
# это не обязательно должно быть главное окно
# может быть просто виджет
class MainWindow(QtGui.QWidget):  
    def __init__(self):
        super(MainWindow, self).__init__()
        self.setAcceptDrops(True)
        
    def dragEnterEvent(self, event):
        event.acceptProposedAction()
        
    def dragMoveEvent(self, event):
        event.acceptProposedAction()
        
    def dragLeaveEvent(self, event):
        event.accept()
        
    def dropEvent(self, event):
        mimeData = event.mimeData()
        if mimeData.hasUrls():
            files = [unicode(url.toLocalFile()) for url in mimeData.urls()]
            event.acceptProposedAction()
            print files # можно делать что-то с именами файлов
            
 
if __name__ == "__main__":
    app = QtGui.QApplication(sys.argv)
    mainwindow = MainWindow()
    mainwindow.resize(300, 300)
    mainwindow.show()
    sys.exit(app.exec_())

 import sys, os

files = [url.toLocalFile() for url in mimeData.urls()]

import sys
from PyQt4 import QtCore, QtGui
import xml.etree.ElementTree as etree
try:
    _fromUtf8 = QtCore.QString.fromUtf8
except AttributeError:
    _fromUtf8 = lambda s: s
    
class Ui_MainWindow(object):
…..

# WARNING! All changes made in this file will be lost!

# -*- coding: utf-8 -*-
import sys
from PyQt4 import QtCore, QtGui
 
from ui_main_window import Ui_MainWindow
 
 
class MainWindow(QtGui.QMainWindow, Ui_MainWindow): # или QWidget
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent)
        self.setAcceptDrops(True)
        self.setupUi(self)
...