SA version is 2.63. And I tried setting report_safe to 0, 1, and 2 and nothing changed. I changed it in /etc/mail/spamassassin/local.cf and my ~/.spamassassin/user_prefs, even restarting the daemon between changes (not sure if it was needed but wanted to be sure).
man Mail::SpamAssassin::Conf mentioned that report_safe needs to be used for anything beyond _HITS_ and _REQD_ tags, but I didn't see anything else that would be a requirement. The output from spamc shows that report_safe seems to be set to 1, but as I said, if I change it to 0, 1, or 2, it still adds the original message as an attachment.
If changing report_safe to 0 doesn't stop SA from encapsulating messages you have one of the following problems:
1) you forgot to restart spamd (you have to do that any time you change local.cf)
2) you have a typo in your file and forgot to run spamassassin --lint
3) you edited a different local.cf than SA is using, check spamassassin -D --lint to see where the "site rules" dir is.
4) you're using an integration tool like mailscanner that does it's own encapsulation, regardless of how SA is set up.