code: function InitiateMultipartUpload(fileUpload, awsKey) {
              var request = {
                  method: 'POST',
                  path: '?uploads',
                  step: 'initiate',
                  x_amz_headers: fileUpload.xAmzHeadersAtInitiate,
                  not_signed_headers: fileUpload.notSignedHeadersAtInitiate,
                  response_match: '<UploadId>(.+)<\/UploadId>'
              };
  
              CancelableS3AWSRequest.call(this, fileUpload, request);
              this.awsKey = awsKey;
          }
docs: see: http://docs.amazonwebservices.com/AmazonS3/latest/API/mpUploadInitiate.html