We'd probably want to leave FORM method=GET behavior just the way it
is, and update method=POST.
Support on the server side is easy: CGI scripts accepting POSTs could
just start branching on the content-type, e.g., processing a new type
multipart/www-form. A day's worth of hacking.
On the client side, generating multipart/www-form isn't much work
(factoring out the user interface complexity of supporting image
scribbles, etc.), but backwards-compatibility is a bear. The only
way a server can declare format preferences in advance is through
anchors, really; so the only alternative I see is something like
a FORM method=POSTMP.
Dave, do you want to try this with your browser? We'll support it
on the server side.