Idea: Add support for private attachments to an article

Justification: We often export an MS Office or exported Google Docs document to PDF and then attach the PDF to an article. When it is necessary to edit the content of that PDF later, the source article is often hard to find or even erased after the original author left, requiring that we recreate the article from scratch.

It would be so much easier if we had the option to save the source document in its original form as a private attachment to the article so we don't have to go on a hunting expedition to find it. Using the "Team notes" field to identify the source is helpful, but depending on account changes to the original repository, sometimes the source is gone.