Hi Anke,
Your extension with disabled notification unofortunatelly didn't work for me. Anyway I added:
<uses-permission android:name="android.permission.DOWNLOAD_WITHOUT_NOTIFICATION" />
in the manifest file of my apk, installed apk but the notification still appears. What is more interesting I have apk file of other application which was made in MIT app inventor and which does not have this specific permission in manifest but despite this fact it is not showing download notification. So there has to be different way of getting rid of this notification. Any ideas? 