title: Stop Blob URL from expiring
body: <p>I am uploading images to the blob store.  I have copied the example 
  from 
  <a href="http://code.google.com/appengine/docs/java/blobstore/overview.html" rel="nofollow">here</a>. </p>
  
  <p>The only problem I encounter is: If I load the page with the form, and 
  not immediately submit the image.  The URL can expire and when I do 
  try and load the image I get an error page.  How can I check to see if 
  the URL has expired and refresh the URL without the user knowing? </p>