Cannot set properties on QJSEngine's global object in initializeEngine()

void IopluginPlugin::initializeEngine(QDeclarativeEngine *engine, const char *uri)
{
    QObject *fileObject = new QObject();

    QJSValue fileValue = engine->newQObject(fileObject);
    engine->globalObject().setProperty("File", fileValue);

    qDebug() << "File value in the global object is:" << engine->globalObject().property("File").toString();

    QJSValue str = engine->globalObject().property("String");
    str.setProperty("File", fileValue);

    qDebug() << "File value in String object is:" << str.property("File").toString();
}

File value in the global object is: "" 
File value in String object is: "QObject(0x102501c60)"