code: function(id, xhr, chunkIdx) {
                  var response = upload.response.parse(id, xhr),
                      etag;
  
                  if (response.success) {
                      etag = xhr.getResponseHeader("ETag");
  
                      if (!handler._getPersistableData(id).etags) {
                          handler._getPersistableData(id).etags = [];
                      }
                      handler._getPersistableData(id).etags.push({part: chunkIdx + 1, etag: etag});
                  }
              }
docs: The last step in handling a chunked upload.  This is called after each chunk has been sent. The request may be successful, or not.  If it was successful, we must extract the "ETag" element in the XML response and store that along with the associated part number. We need these items to "Complete" the multipart upload after all chunks have been successfully sent.