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