title: Get around tkinters' askopenfilename filetype check
body: <p>I'm using tkinters' askopenfilename with a handful of predefined filetypes:</p>
  
  <pre><code>tkinter.filedialog.askopenfilename(parent=self,
                                     title=_("Select Attachment"),
                                     multiple=False,
                                     initialdir=initialdir,
                                     filetypes=filetypes)
  </code></pre>
  
  <p>filetypes include 'png', 'jpg', 'bmp', ...</p>
  
  <p>The filedialog will only show me files of the specified type as I expected.
  But when I type in the file name it will just accept it, no matter of which type it is.</p>
  
  <p>Is this a bug of tkinter? Is it expected behavior? Do I need a workaround or can I change askopenfilenames' behavior?</p>