Unit testing BlobStore create_upload_url uploads

I'm using the using the GAE BlobStore API to create an upload URL, as follows:

self.response.write(blobstore.create_upload_url(....))

Everything works fine in production, but when using testbed to unit test this API, I get the following URL back:

http://ift.tt/1DMfjgs

Uploading to this URL from the test doesn't work, I get a 404. I should have initialized all stubs properly, amongst others:

self.testbed.init_datastore_v3_stub()
self.testbed.init_blobstore_stub()
self.testbed.init_files_stub()

What am I doing wrong? How do I unit test file uploads using the blobstore create_upload_url API?