nuxeo-ai doesn't support an s3 bucket prefix

Given a Nuxeo deployment, using S3, and using a prefix for the S3 storage, the AI.Encrichment operation can't access the binary with an error like:

Caused by: com.amazonaws.services.rekognition.model.InvalidS3ObjectException: Unable to get object metadata from S3. Check object key, region and/or access permissions. (Service: AmazonRekognition; Status Code: 400; Error Code: InvalidS3ObjectException; Request ID: e052d696-5b05-11e9-9042-edcb74beee08)

If you remove the prefix, it works ok.