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 = ..... 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)