When Mail Snapshots Attachments
In Mail, the encoding of the attached file takes place immediately when you attach the file. In other words, when you attach a file to an e-mail message in Mail, Mail actually copies the file in encoded form into the body of the e-mail message. If, after that, you leave your message window open and return to your original file in its parent application and continue editing and saving it, this will not affect the version of the file that’s included in the e-mail message that you’ve left open in Mail.
In a vacuum, I think a good case could be made that Mail 2.0’s behavior is correct, but it differs from the way Mac mail programs have traditionally worked.
Actually, I can't count how many times I removed a attachment from a mail in order to modify it, just because I couln't answer this question.
I'm quite happy I've got an answer now, no matter which one.