onSuccessFromServer = function(successRequestResult) {
                    delete self._failedSuccessRequestCallbacks[id];
                    qq.extend(result, successRequestResult);
                    qq.FineUploaderBasic.prototype._onComplete.apply(self, onCompleteArgs);
                    promise.success(successRequestResult);
                }
When the upload has completed, if it is successful, send a request to the `successEndpoint` (if defined).
This will hold up the call to the `onComplete` callback until we have determined success of the upload
according to the local server, if a `successEndpoint` has been defined by the integrator.

@param id ID of the completed upload
@param name Name of the associated item
@param result Object created from the server's parsed JSON response.
@param xhr Associated XmlHttpRequest, if this was used to send the request.
@returns {boolean || qq.Promise} true/false if success can be determined immediately, otherwise a `qq.Promise`
if we need to ask the server.
@private