It seems that the Amazon S3 support for file storage is broken. It is likely that this is due to the changes in Amazon side. Right now the default AWS SDK for Python is boto which is already supported by django-storages. it only requires to specify a different backend.

This will add a new dependency to reviewboard in order to support Amazon S3. Apart from django-storages, boto package is also required.

Note: I needed to uninstall the botocore package which was also installed in my system. It seems there was a conflict with boto.

Apply the patch. Install django-storages and boto with pip (also uninstall botocore which I had installed). Configure an S3 bucket with subdomain path. Create a new request and upload an screenshot. The request is created and the image is placed in the specified bucket. 

The test has been done and works with ReviewBoard 1.6.3 and ReviewBoard 1.7.11