Uploading multiple files with will_paginate

hey all

i'm using attachment_fu to add 'documents' (the attached class) to a
forum post (in our app). It's all working fine for a single document -
on the new post page i just have a file_field which sets
"post[document]", which calls my Post#
document= method, which builds a document for that post:

  def document=(file)
    self.documents.build(:uploaded_data => file)
  end

Then the post is saved and it's all good.

I thought it would be nice to be able to add multiple documents at once.
So, instead of writing to "post[document]" in the form, i'm writing to
"post[documents][]", and i get an array of files through. I then do
this -

  def documents=(files)
    files.each{|file| self.document = file}
  end

expecting that this would build three documents, and that when i saved
the post then the documents would get created. However, i'm getting
this error, when i try to do the Post.build(params[:post]) in the
controller:

Document(#-635202458) expected, got
ActionController::UploadedTempfile(#-613987618)

Is it the case that there's some rails magic going on here, and we're
not really dealing with file objects after all, but a tempfile, of which
there can only be one? (like in highlander) Or something else?

Grateful for any guidance...
max