code: function ReuseS3Object(fileUpload, awsKey) {
              this.awsKey = awsKey;
  
              fileUpload.info('will attempt to verify existence of the file');
  
              var request = {
                  method: 'HEAD',
                  path: '',
                  x_amz_headers: fileUpload.xAmzHeadersCommon,
                  success404: true,
                  step: 'head_object'
              };
  
              SignedS3AWSRequestWithRetryLimit.call(this, fileUpload, request);
          }
docs: http://docs.amazonwebservices.com/AmazonS3/latest/API/mpUploadComplete.html