Inline file upload error: undefined method `size='

Hello,

I followed the tutorial at:
http://khamsouk.souvanlasy.com/2007/5/1/ajax-file-uploads-in-rails-using-attachment_fu-and-responds_to_parent

However, in my controller I am getting the error:

NoMethodError (undefined method `size=' for #<SwfImage:0x5705b48>)

Other references to this error indicate that the render method is
trying to render a string (which does have a size method) but it's
attempting to render the SwfImage object instead. So how would I get
around this? Any help would be appreciated.

#### VIEW CODE ####

<% form_for(:swf_image, :url => formatted_swf_images_path(:format =>
'js'), :html => { :multipart => true, :target => 'upload_frame' }) do |
f> %>
  <p>
    <b>Filename</b><br />
    <%= f.file_field :uploaded_data %>
  </p>

  <p>
    <%= f.submit "Create" %>
  </p>
<% end %>
<iframe id="upload_frame" name="upload_frame" style="width:1px; height:
1px; border:0px;" src="about:blank"></iframe>

#### CONTROLLER CODE ####

  def create
    @swf_image = SwfImage.new(params[:swf_image])

    respond_to do |format|
      if @swf_image.save
        flash[:notice] = 'Image was successfully saved.'
        format.html { redirect_to(@swf_image) }
        format.js do
          responds_to_parent do
            render :update do |page|
              page.insert_html :bottom, "swf_images", :partial =>
'swf_images/image', :object => @swf_image
              page.visual_effect :highlight, "image_#{@swf_image.id}"
            end
          end
        end
      else
        format.html { render :action => "new" }
        format.js do
          responds_to_parent do
            render :update do |page|
              # do nothing
            end
          end
        end
      end
    end
  end