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.