I have prepared a demo app, based upon the previous one, that includes a file picker. This has been tested with companion on my Genymotion Emulator, using Android 9 and 10 - it works fine on both android versions.
Let me know how you get on with it.
Gratitudes: 
to @Taifun for the File Extension and @Juan_Antonio for the Base64 Extension, without whom, and their good work, none of this would be possible 
AIA File
UAFTGD_FilePicker.aia (103.5 KB)
(you will need to add the url to your google apps script)