My designers post files to a private message thread for review internally by me (creative director) or a project manager. If the file’s okay, we need to get it approved by our client. But Basecamp doesn’t seem to let us make the file public without making the parent thread public at the same time. It would be great if we could “detach” the file and attach it to a new public message.

We use a similar process.
Being able to link an already uploaded file, from the file section, to a message or a comment would be fantastic.

I also have a need for this type feature. I often find myself downloading private files only to turn around and upload them again. Filling the database with duplicate files.