When editing the header of your outgoing message, there are a couple of special features available.
If you specify
Mutt will pick up filename just as if you had used the edit-fcc function in the compose menu.
You can also attach files to your message by specifying
Attach: filename [ description ]
where filename is the file to attach and description is an optional string to use as the description of the attached file.
When replying to messages, if you remove the In-Reply-To: field from the header field, Mutt will not generate a References: field, which allows you to create a new message thread.
Also see edit_headers.