I was proposing to use the file extension (.xbm above) to tag what format the image was in, but with the intention that in future, when HTTP2 comes along, the same format negotiation technique would be used to access images.
Nineteen years later and we still don't have HTTP2.
“HTTP2” is a reference to Basic HTTP as defined in 1992. At this point, in early 1993, it was still largely unimplemented. The draft known as “HTTP2” evolved and was eventually standardized as “HTTP 1.0” (albeit not for another three years). HTTP 1.0 did include request headers for content negotiation, a.k.a. “MIME, someday, maybe.”
So, at least in this context, yes, we do have "HTTP2".
Nineteen years later and we still don't have HTTP2.