What is not clear in my answer? You wrote that ASD is disappearing after disinstalation. Its obvious. But it's also obvious that once reinstalled an app on this or new smartphone - the app will recreate an ASD and can restore all user data from backup file saved on SD card in SD reader.
I've done it already for small files. It works.
The crucial question is who is modifying the file in question (.zip)? If this isn't the user then the whole problem should be easily solvable - if the problem ever existed.
Sorry, Anke. I can't understand you. Why do somebody need to modify a backup file?!
I have only one problem - how to copy big file from the SD card in SD reader to ASD? That's all. Is it possible to receive an answer from the developer of SAF about related sponsored upgrade?
Sorry, but now I'm going to sleep. It's already about 2 o'clock here 
Yes, offcourse.
I already asked that: Who creates the zip file and who changes it and is it changed at all? What is your app about? It's always the same questions... and that's why I made my guide "How to ask a question".
Maybe I was not able to explain - it's a backup file and it was created by my app in ASD. But as a backup it has to be saved in secured place that gives possibility to restore this important data in case of smartphone damage or other problems. The solution is to save it on sd card reader connected via USB that is much more secure than the smartphone or cloud.
The user can save this backup on SD card in SD reader with a help of sharing function and it works. But I need to give him possibility to restore this backup file copying it back to ASD from SD card in SD card reader.
I can do it now with small backup files (up to 5mb) with SAF. But backup file can easily arrive to 100-200 mb and in this case SAF is giving a memory allocation error.
That's why I'm asking the developer of SAF to "upgrade" it in order to support big files.
If you can do it - what are the next steps? It will be my first experience as a sponsor
Backup file is a backup file and its obvious that it can't be changed. Never and by nobody.
Have you ever changed a backup that you were producing on your PC?
The app is producing it and the app is restoring it. As is.
How about explaining what your app is actually about? I do not understand anything anymore.
A backup of what???
That's why I point out in my guide to communicate what exactly the goal of the app is.
Sorry Anke. But it seems that you are not reading my answers. My app is saving and working with important user data. That's all.
I'm absolutely sure that the developer of SAF has understood very well the problem and it seems that he is ready to implement an upgrade. Could you please explain me how to proceed with a sponsored upgrade from the organisation point of view?
I have read it carefully and should be able to understand what the app is actually about.
How about if you explain it clearly. Where does the .zip file come from and who can change it?
Are these really too difficult questions?
I've answered you already many times!? But I'll try againš
The backup of all data saved by user in my app is produced by my app as an encrypted zip file on the external SD card and in case of necessity it has to be restored from the external SD card by my app.
But please help me with understanding of next organisational steps with sponsored upgrade? Do you have a separate communication channel with a developer?
Ok, the user saves data / files in your app. Fine. Which data and files? How about if we could learn something from you and your app.
So again, what is your app about? Can you please answer this banal question?
Anke, I need an answer to my question and I don't need a marketing support for my app promotion 
What was written by me is more than enough for the explanation of my problem and the developer has understood perfectly what is necessary to do.
Could you please answer to my question - do you have a non public channel with a developer for the discussion of the sponsorship?
If you ask questions here in the forum, it is always interesting to know what the problem is about in general. Basically, these are questions that could affect everyone.
So we'd like to understand what your app is all about.
Is this simple question really that difficult to answer?
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Anke, have you ever worked in Putin's KGB or in the CIA? Surely you could arrive to the very top of those organisationsš¤.Who can resist such pressure?!
My application gives possibility to save inside it and ONLY on you own Smartfone all your passwords, documents, secrete pictures and so on. All important info is encrypted (AES 256 bit) and zipped. You can easily query those data and view documents but only from the app. The main difference of this app is that nothing is transferred to the cloud or to other third parties including the developer of the app. In this way guys from the organizations mentioned above or other harkers will have more problems to read your data. The app is free of charge because... I'm already on the retirement and its my hobby.
But please don't tell it to KGB guys!
P.s.Backup and restore functionally of all those data is the foundation of such app.
Ahh, now I see why you beat around the bush for so long.
So what exactly does the app store and how much data and to what extent?