Thanks both of you. My problem has been solved with a simple "work-around". In the same forum, or in the same thread, or even in the same posting I am able to control which file is available to everyone, and which file is available to only registered members.
1) Everyone (even unregistered members) may download a file if the file is linked by a hyperlink, for example
"download here"
2) But only registered members (if permission is set accordingly) will be able to download a file, if that file has been uploaded using the "Manage Attachments" feature.
This completelly achieves what I needed