Rails - May 2006 - form_for and Multipart - Syntax?

I am trying to get into the habit of using form_for in my new applications
but I can''t seem to figure out how to get it to make my form use
multipart
encoding.

I''ve tried adding :multipart => true in various spots to no avail.
The
source reads like Greek to me. Can someone help out with the correct syntax,
it''s not in the doco.

Thanks.