Can't copy description with attachments when authenticated as addon app

[Ania]

Hi, I have a post function which copies description from subtask to main issue and everything works perfectly fine until it contains attachments, then I get an error:

500 Internal Server Error: [{"errorMessages":["We couldn't save your changes. Copy your content, then try reloading the page."],"errors":{}}]

But when I send the same request authenticated as a user, not as an addon, it works and description is copied with attachments as expected. How can I make it possible for addon?

I checked in Issues > Permission schemes that permission for attachments is already granted to atlassian-addons-project-access, but it doesn't work anyway.

Edit: To clarify, it works only if request is sent by the user who also added the attachment.