title: mimetypes.mime_guess() on google app engine behaves strange
body: <p>In my python shell, i can do</p>
  
  <pre><code>&gt;&gt;&gt; import mimetypes
  &gt;&gt;&gt; mimetypes.guess_type("ulla.svg")
  ('image/svg+xml', None)
  </code></pre>
  
  <p>And it behaves as expected, however, running the same code (or at least, this equal example) on google app engine, it returns <code>(None, None)</code></p>
  
  <pre><code>class TestHandler(webapp2.RequestHandler):
      def get(self):
          import mimetypes
          self.response.out.write(mimetypes.guess_type("ulla.svg"))
  </code></pre>
  
  <p>Am i doing it wrong? :)</p>
  
  <p><strong>BTW</strong> - It's python 2.7 in my macbooks shell, and also 2.7 on app-engine</p>