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 	// form https://www.raywenderlich.com/1047-advanced-collection-views-in-os-x-tutorial
    let openPanel = NSOpenPanel()
    openPanel.canChooseDirectories = false
    openPanel.canChooseFiles = true
    openPanel.allowsMultipleSelection = true;
    openPanel.allowedFileTypes = ["public.image"]
    openPanel.beginSheetModal(for: self.view.window!) { (response) -> Void in
      guard response.rawValue == NSFileHandlingPanelOKButton else {return}
      self.insertAtIndexPathFromURLs(urls: openPanel.urls, atIndexPath: insertAtIndexPath)
    }