Make sure you are using application/json as the content type for your request
Thanks Nils, yea I've got the content type as application/json for the AJAX request. Although I was just looking into the Jive code for this end point (com/jivesoftware/api/core/v3/services/content/AttachmentService.java) and it looks like maybe it only accepts multipart/form? So maybe for an existing doc you can't upload an attachment for a URL like this?? Seems weird if that's the case because it works fine with creating a new piece of content.
public class AttachmentService extends AbstractService
protected AttachmentProvider attachmentProvider;
protected CommonContentProvider commonContentProvider;
protected MessageProvider messageProvider;