Ah! The "View Soure" capability likely does not employ any kind of MIME processing. This would make it similar to a "raw" output.

I tried this on my Mozilla Mail client, and indeed I saw all of the attachments as you describe. IMHO this is more to do with the browser/client than the MIME protocol. In fact, back in the days when I used GNUS in EMACS to read my email, it behaved this way by default.

HTH
--
idnopheq
Apply yourself to new problems without preparation, develop confidence in your ability to to meet situations as they arrise.

~ Shunryu Suzuki