Expanded Replacement Tokens

The standard attachment replacement token only includes the attachment ID, and is implicitly assumed to be tied to the current message / channel. However if you disable Reattach Attachments in the Destination Settings, the destination will replace the token not with the actual attachment data, but instead with an expanded token:

The expanded token contains the channel ID, message ID, and attachment ID, so that you can uniquely identify an attachment even from a completely different channel or message. Because of this, you can use this replacement token in downstream channels and reattach attachments from earlier, upstream channels.