How can I let users access a specific attachment?

WordPress (or more precisely its .htaccess or alternate web server configuration file) passes through requests to existing files.

The easiest way would be to start at root of your site and (using FTP/SFTP/hosting admin panel/etc):

  • create blank01 folder
  • create blank02 subfolder in it
  • place file.xml into subfolder

And it should just work, link will point to real file and WP won’t get involved at all.