onGetKeynameSuccess = function(keyname, extension) {
                    var keynameToUse = keyname;

                    if (extension !== undefined) {
                        keynameToUse += "." + extension;
                    }

                    promise.success(keynameToUse);
                }
Determine the file's key name and passes it to the caller via a promissory callback.  This also may
delegate to an integrator-defined function that determines the file's key name on demand,
which also may be promissory.

@param id ID of the file
@param filename Name of the file
@returns {qq.Promise} A promise that will be fulfilled when the key name has been determined (and will be passed to the caller via the success callback).
@private