title: Google App Engine: upload file and other fields in the same request
body: <p>I want to upload an image to the blobstore, because I want to support files larger than 1MB. Now, the only way I can find is for the client to issue a POST where it sends the metadata, like geo-location, tags, and what-not, which the server puts in an entity. In this entity the server also puts the key of a blob where the actual image data is going to be stored, and the server concludes the request by returning to the client the url returned by create_upload_url(). This works fine, however I can get inconsistency, such as if the second request is never issued, and hence the blob is never filled. The entity is now pointing to an empty blob.</p>
  
  <p>The only solution to this problem I can see is to trigger a deferred task which is going to check whether the blob was ever filled with an upload. I'm not a big fan of this solution, so I'm guessing if anybody has a better solution in mind.</p>