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