Why is attachment_display after content length header?

[XTF]

Hi,

I'd like this hook to modify the attachment before it's send to the client, but I can't because the content length header is already send and the length will change.
Why is this hook after and not before the content length and other headers are send?

[Princeton]

That doesn't sound right...
you want to modify the attachment each time it's delivered to the end-user?

Why would you want to do that?

Modifying attachment should be done when "uploading" attachment occurs OR via a cron job.

[XTF]

That doesn't sound right...
you want to modify the attachment each time it's delivered to the end-user?

Why would you want to do that?

Modifying attachment should be done when "uploading" attachment occurs OR via a cron job.
Yes, I do. I want to insert a per-user ID in the data.