I found it!
The problem was that I didn't mentioned URL to update.plist in my info.plist (which is part of extension). I simple didn't know, that I have to. I mentioned this URL in submission form and expected that this is enough.
I checked the docs, and it literally says "include the URL of the file in the Update Manifest field of Extension Builder". If only it says instead: "add URL of the file to your info.plist, which is part of your extension" I would definitely did it. But, as soon as I personally didn't develop extension (I give this work to another programmer in my team), I confused "Extension Builder" with submission form.