How to set group header via VK API?

Question

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For starters, here's what a group header is:

The API documentation says absolutely nothing about loading. Forays through unofficial clients no longer help, because they do not implement this (reference to this question). By looking at the requests, I found that the call signature does not look like any of the file upload methods/methods listed here . So, what I was able to unearth from the requests:
1. Each file uploaded to the header has a _sig. It strangely changes from request to request, but without it you can't download the file.
2. If we catch VK requests, then we will get a static link to download the file. This static link works fine and when you go to it, it says this:

{"error":"ERR_UPLOAD_FILE_NOT_UPLOADED: file not found","bwact":"owner_cover","server":836521,"mid":151665536,"_sig":"83447078c5079124026b80d90f116398"}

However, after executing the download in the code, we get:

{"error":"ERR_UPLOAD_BAD_SIGNATURE","bwact":"owner_cover","server":836521,"_sig":"d762c16dc795c4f65b59aafdaa39628a"}

3. The method used by VK (usually the method is visible in the act parameter . Let's say, when downloading an audio message , it was add_doc ), is missing in the documentation and looks like this: act=owner_cover .
Who is interested - please help. Thanks in advance for your answers! If you have any ideas - they can also be in the answers or comments. Perhaps the question will also grow like this one ...
P/s The question is duplicated here: ru.stackoverflow.com/q/613687/219079

Answer 1

The first POST request goes to https://vk.com/al_page.php
parameters: act=owner_photo_box&al=1&cover=1&oid=-85297730
https://gist.github.com/S-anasol/643d82f34ca6518d4...
The response will be code with an upload initializer and it will contain a link including the first _sig
Further, _sig changes from request to request, there is one more involved, you get it after the first upload.

Answer 2

When I had similar problems with the VK API, I turned to support for help.
I'm sure they will help in this case too.
PS: if the issue is not resolved - in the evening I'll dig into the loading of caps, I'll try to help.