title: Uploading file in Rails gives String filename instead of File or StringIO object
body: <p>Rails 3, JRuby 1.6.7.2</p>
  
  <p>I've been trying something "elementary", just uploading a single text file via a form for processing in my app.  The problem I'm seeing is that instead of a StringIO or File, I'm getting just a string of the file name.  </p>
  
  <p>Here's the form code</p>
  
  <pre><code>= form_tag(:controller =&gt; "api/#{CURRENT_API_VERSION}/api", :action =&gt; 'file', :method=&gt; :post, :multipart =&gt; true) do
      = label_tag "file"
      = file_field_tag "upload[file]"
      = submit_tag 'Analyze!'
  </code></pre>
  
  <p>And the controller code that is just giving me @upload as a string containing the file name.  </p>
  
  <pre><code>def file
          @upload = params[:upload][:file]
          render :template =&gt; 'api/file.html.haml'
        end
  </code></pre>
  
  <p>Running debugger in the controller gives me @upload.class = String, and it doesn't respond to any file or StringIO methods, such as read.  </p>