code: function (callback) {
      var partBuffer = preparePartBuffer();
  
      var localPartNumber = partNumber;
      partNumber++;
      receivedSize += partBuffer.length;
      cachedClient.uploadPart(
        {
          Body: partBuffer,
          Bucket: destinationDetails.Bucket,
          Key: destinationDetails.Key,
          UploadId: multipartUploadID,
          PartNumber: localPartNumber
        },
        function (err, result) {
          if (err)
            abortUpload('Failed to upload a part to S3: ' + JSON.stringify(err));
          else {
            uploadedSize += partBuffer.length;
            partIds[localPartNumber - 1] = {
              ETag: result.ETag,
              PartNumber: localPartNumber
            };
  
            callback({
              ETag: result.ETag,
              PartNumber: localPartNumber,
              receivedSize: receivedSize,
              uploadedSize: uploadedSize
            });
          }
        }
      );
    }
docs: Flush a part out to S3.