title: Rails param["value"] is nil
body: <p>For some reason, one of my params (file upload) always is nil. Here is the params hash in the exception page that is shown:</p>
  
  <pre><code>{"utf8"=&gt;"âœ“",
   "_method"=&gt;"put",
   ...
  
   "avatar"=&gt;#&lt;ActionDispatch::Http::UploadedFile:0x007fe7fa1dd460 @original_filename="rp.jpg",
   @content_type="image/jpeg",
   @headers="Content-Disposition: form-data; name=\"user[avatar]\"; filename=\"rp.jpg\"\r\nContent-Type: image/jpeg\r\n",
   @tempfile=#&lt;File:/var/folders/44/xrz6nqg91_l_gzmhxt8s771c0000gn/T/RackMultipart20111107-12870-nguhuw&gt;&gt;},
   "commit"=&gt;"Update",
   "id"=&gt;"1"}
  </code></pre>
  
  <p>In my controller, I have <code>if params["avatar"]</code>, but avatar is always returning nil.</p>