Neat. Though for my own sanity, I'd probably use a file extension like .image for URLs that return different content type based on the Accept header. (And leave .png and .webp untouched so I have the option to use those explicitly)
TBL wrote about this in 1998: http://www.w3.org/Provider/Style/URI.html#remove
(That might be called a browser bug, actually; it'd sure be nice if browsers on these OSes could take the observed media type and tack on an extension to extensionless files on Save As to ensure they'll be viewed by the OS to have that same media type. Or it might be called an OS bug: why the heck don't filesystems have a required media-type metadata field for files yet!? ;)