'PUT' transaction reconsidered (was Re: two-phase send concerns )

I'm becoming more and more convenced that single-exchange PUT is
unworkable. That is, PUT might have to happen in two steps:



1. client: wanna PUT this file of this size of this media type 
  	   in this location with these PERMS with
	   this validator 
2. server: OK, send file (or permission denied)
3. client: OK, here's the file.
4. server: OK, got the file

Step 1 might offer more than one media type with step 2 accepting a
particular one or a subset. Step 1 might say that it doesn't know the
file size, for example.

Opinions?