class LoadDialog(FloatLayout):
    load = ObjectProperty(None)
    cancel = ObjectProperty(None)

<LoadDialog>:
    BoxLayout:
        size: root.size
        pos: root.pos
        orientation: "vertical"
        FileChooserListView:
            id: filechooser

        BoxLayout:
            size_hint_y: None
            height: 30
            Button:
                text: "Cancel"
                on_release: root.cancel()

            Button:
                text: "Load"
                on_release: root.load(filechooser.path, filechooser.selection)

def show_load(self):
        content = LoadDialog(load=self.load, cancel=self.dismiss_popup)
        self._popup = Popup(title="Load file", content=content,
                            size_hint=(0.9, 0.9))
        self._popup.open()

def load(self, path, filename):
        wimg = os.path.join(path, filename[0])
        self.image_source = wimg
        self.dismiss_popup()