S3 direct upload fails silently because there is no policyTemplate contributed by default

When trying to upload something, an user gets a 403 on the PUT on the bucket but no indication on what's the problem in the logs.
After debugging, it turns out that S3 direct upload expects a contributed policy template that will allow "s3:PutObject" on the transient store.
This policy should exist by default, since its content it pretty much the same all the time ( the most we should have the right to override it if needed).