Thanks for the pointer! I took the opportunity to learn a bit about more recent NNTP by reading the standard: RFC 3977. It looks like nntp v2 circa 2006 added MIME encoding, so I would guess that may be how a service provider would differentiate.

I haven't used Usenet since the turn of the century. Back then it was all text (including every article under alt.binaries), and even pirated media needed to be split into a multi-part format (often rar) then each part uuencoded so it could be included in an article.