For e-mails, secure uploads redacts all secure images, and later uses the access control post to re-attached allowed ones. We pass the ID of this post through the X-Discourse-Post-Id header. As the name suggests, this assumes there's only ever one access control post. This is not true for activity summary e-mails, as they summarize across posts.
This adds a new header, X-Discourse-Post-Ids, which is used the same way as the old header, but also works for the case where an e-mail is associated with multiple posts.
f3cad5f3a2