Hi Matze,
It's nice to see you are still giving OneNote a chance :-) Of course i looked into the Android client option. I am running OneNote on a Samsung Note tablet with Android on it. Opened OneNote and went on a search. I have a couple of Notebooks which contain the word Excel. So i tried a search for this word and have to say that my search results were based on all notebooks available. So where do we differ here?
As for the file upload you are right there. This is client specific and only available from there. A sneeky workaround might be adding a link and let that link refer to your file.