code: function handleInitiateRequestComplete(id, xhr, isError) {
          var promise = pendingInitiateRequests[id],
              domParser = new DOMParser(),
              responseDoc = domParser.parseFromString(xhr.responseText, "application/xml"),
              uploadIdElements, messageElements, uploadId, errorMessage, status;
  
          delete pendingInitiateRequests[id];
  
          // The base ajax requester may declare the request to be a failure based on status code.
          if (isError) {
              status = xhr.status;
  
              messageElements = responseDoc.getElementsByTagName("Message");
              if (messageElements.length > 0) {
                  errorMessage = messageElements[0].textContent;
              }
          }
          // If the base ajax requester has not declared this a failure, make sure we can retrieve the uploadId from the response.
          else {
              uploadIdElements = responseDoc.getElementsByTagName("UploadId");
              if (uploadIdElements.length > 0) {
                  uploadId = uploadIdElements[0].textContent;
              }
              else {
                  errorMessage = "Upload ID missing from request";
              }
          }
  
          // Either fail the promise (passing a descriptive error message) or declare it a success (passing the upload ID)
          if (uploadId === undefined) {
              if (errorMessage) {
                  options.log(qq.format("Specific problem detected initiating multipart upload request for {}: '{}'.", id, errorMessage), "error");
              }
              else {
                  options.log(qq.format("Unexplained error with initiate multipart upload request for {}.  Status code {}.", id, status), "error");
              }
  
              promise.failure("Problem initiating upload request.", xhr);
          }
          else {
              options.log(qq.format("Initiate multipart upload request successful for {}.  Upload ID is {}", id, uploadId));
              promise.success(uploadId, xhr);
          }
      }
docs: Called by the base ajax requester when the response has been received.  We definitively determine here if the
  "Initiate MPU" request has been a success or not.
  
  @param id ID associated with the file.
  @param xhr `XMLHttpRequest` object containing the response, among other things.
  @param isError A boolean indicating success or failure according to the base ajax requester (primarily based on status code).