Comment 1 for bug 512085

We've found that users are very poor at separating out information. If we have public and private attachments on one bug, its almost certain someone will paste unredacted content from a secret attachment onto the bug, thereby defeating the private setting on it - and the UI for individual attachments is much narrow than for the entire bug so its going to be harder to get it right there.

Accordingly, I think this would be a misfeature, it would encourage something we want to avoid. The upcoming bug linking feature may address the source of your complaint - that bugs with a public oot cause end up private because of the data needed to analyse them. (Using a dependency bug will let this be pretty clean).