> When a user sends an attachment (e.g., an image) on Signal, it is uploaded to cdn2.signal.org.

Why is that even the case? I had understood that (binary) attachments are embedded into the encrypted message and hence transferred directly from sender to receiver.

Obviously, retrieving media from an external location saves bandwidth at multiple positions. I am not a security expert, but it seems almost trivial to see how storing message data on an external server conceptually facilitates attacks like this one. Isn't that the same reason a link preview is generated at the sender first and then embedded into the message as an image?