Yes, this isn't really for images. It's useful for little bits of
text, footnotes, references, and odd media types that really just are
the parameters for helper apps. It makes no sense to support this
without also supporting the fuller multipart/related and cid:
mechanism for included references to larger data.
One of the primary applications for 'data:' is for dealing with
supplying the parameters for another helper app, in the same way that
'telnet://host.dom/' on the Mac actually winds up creating a little
parameter file (application/x-telnet-parameters) which is then
interpreted with the telnet application.
If it were possible to configure browsers to deal with local URL
schemes in the same way, this might not be necessary; you'd just
define a new URL scheme for each media type that you wanted.