title: Does blobstore URL expire? Getting 500 error
body: <p>I've got a fairly new site where I upload images using the blobstore
  and storing the url in my datastore model as an db.StringProperty().</p>
  
  <p>When the file is uploaded I get the serving url with: <code>image.imageUrl = images.get_serving_url(str(upload_files[0].key()))</code> where image is my datastore model.
  I haven't modified the code for about 2 weeks and it's been working so far.</p>
  
  <p>But all of the sudden all url's just returns 500 error when trying to open them.</p>
  
  <p>The URL <a href="http://lh4.ggpht.com/JfA6nTibsx99EJ2TQoQZr5OVXhQUj_tJK0wCP3FJSRq9RhCzUFLAwsR1q0bzplmqRaRu6fTnNPqDFzpxzMMBeva9FJ9WRXdAqgA" rel="nofollow">http://lh4.ggpht.com/JfA6nTibsx99EJ2TQoQZr5OVXhQUj_tJK0wCP3FJSRq9RhCzUFLAwsR1q0bzplmqRaRu6fTnNPqDFzpxzMMBeva9FJ9WRXdAqgA</a>
  just returns 500 error. </p>
  
  <p>Some what strange is that adding a resizing (adding =s720) param works:</p>
  
  <p><a href="http://lh4.ggpht.com/JfA6nTibsx99EJ2TQoQZr5OVXhQUj_tJK0wCP3FJSRq9RhCzUFLAwsR1q0bzplmqRaRu6fTnNPqDFzpxzMMBeva9FJ9WRXdAqgA=s720" rel="nofollow">http://lh4.ggpht.com/JfA6nTibsx99EJ2TQoQZr5OVXhQUj_tJK0wCP3FJSRq9RhCzUFLAwsR1q0bzplmqRaRu6fTnNPqDFzpxzMMBeva9FJ9WRXdAqgA=s720</a></p>
  
  <p>Anyone encountered this? Sounds strange the a url would expire.</p>
  
  <p>..fredrik</p>
  
  <p><strong>EDIT:</strong></p>
  
  <p>Reading the documentation I interpret that it should always be the same URL (am I wrong?):</p>
  
  <p><em>The URL returned by this method is always public, but not guessable; private URLs are not currently supported. If you wish to stop serving the URL, delete the underlying blob key. This takes up to 24 hours to take effect.</em></p>