This is an interesting answer if you think that 'client-editing' is
the most common case of a client uploading a file to a server.
However, the kinds of applications that have been proposed are:
NSF puts up web servers for grant applications.
You fill out the form, and, as part of it, submit the
postscript of your application, the individual resumes
of all of the participants, and the budget.
SGML validation service: you supply the DTD as one file,
and the SGML file as another.
etc. etc.
I'll be the first to admit that lots of applications need only a
single file upload, but lots more might want several files. In either
case, no changes to 'HTTP' are being proposed. Changes to HTML,
additional MIME types to encapsulate multiple values more efficiently,
etc.
Submitting things by ftp: or mailto: make perfect sense, too.