file_field and rails3

Hello everybody, i need to manage file uploads with a new rails application. I proceded as i did with rails2 - declared the form as multipart <%= form_for(@photo, :html => {:multipart => true}) do |f| %>

- and added the file_field tag     <%= f.label :uploaded_data %><br />     <%= f.file_field :uploaded_data %>

the form shows up correctly, but after submit i receive an error: 500 Internal Server Error

If you are the administrator of this website, then please read this web application's log file and/or the web server's log file to find out what went wrong.

in the server log i get this: TypeError (can't convert nil into Integer):

the controller is still the one produced by the "rails scaffold", so it should work fine (obviously i don't expect it to really save the file).

What am i missing?

Using: ruby -v ruby 1.9.2p0 (2010-08-18 revision 29036) [x86_64-darwin10.4.0] rails --version Rails 3.0.0