public String onUploadInitiation(UploadObjectRequest req) {
        InitiateMultipartUploadResult res =
            s3.initiateMultipartUpload(newInitiateMultipartUploadRequest(req));
        return this.uploadId = res.getUploadId();
    }
Notified from
{@link AmazonS3EncryptionClient#uploadObject(UploadObjectRequest)} to
initiate a multi-part upload.

@param req
           the upload object request
@return the initiated multi-part uploadId
aws-java-sdk-s3/src/main/java/com/amazonaws/services/s3/UploadObjectObserver.java