title: Parsing multipart form data on the server
body: <p>Suppose I have a form for uploading a file:</p>
  
  <pre><code>&lt;%= form_tag({action: :upload}, multipart: true) do %&gt;
    &lt;%= file_field_tag 'picture' %&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <p>I need to parse the multipart data of that file manually on the server in an action, in particular I need to get its length and name:</p>
  
  <pre><code>def upload
    # ???
  end
  </code></pre>
  
  <p>How do I do that?</p>