multipart post/update without form view

I am using attachment_fu and want to do an update to an attachment.

def update
......
attachment.update_attributes(params[:photo])
....
end

this works fine for a form post to action update

But I want to do this in the controller, where I open a local file and
do the update.

For example:

def myupdate
...
photofile = File.open(...)
.....
attachment.update_attributes(some photo file)
...
end

I've seen some long winded way of creating a multipart file piece by
piece. But is there an easiser way?
(something like a file_upload in the test harness)