Hi @dmitry_bobrov,
When you delete a post, any attached files remain stored on the server (or in S3 if that's being used). If you had version 3.5 or newer, a link from the post to the file would also remain in the database.
We don't have any support for having the server permanently delete file attachments though. If you'd like to file this as a feature idea or show support for a similar idea posted by another user, that would be helpful so we can prioritize based on community feedback.