File upload problem - blank page after submission

Hi,

The form definition below seems to work fine on the "GET" request, but
when I post, control is not passed to the action controller's method
modify. The return render is a blank page. There is nothing in the
development.log file which indicates what could be going wrong.

Firebug shows the submission, and 200 OK respond from webrick.

So I'm lost now, can somebody help understand the blank page, and why
the processing is not passed to the control's method (modify)?

Thanks
Chris

<% form_for(@web_captcha, :url => { :action => "modify" }, :html =>
{:multipart => true, :class => "nifty_form"}) do |f| %>
  <%= f.file_field :image_data %>
  <%= submit_tag "Update" %>
<% end %>

Should that be f.submit "Update" ?

Colin