Upload Brand Article Reference

Attach a file the draft will be written from.

The upload to the model provider happens here, while the marketer is still at the composer, so an unreadable file is refused at attach time rather than surfacing minutes later as a draft that did not finish.

What comes back is the attachment record's opaque id. The provider file id and the storage location stay on the server, so nothing the browser holds can be edited into a claim on another workspace's document.