title: Image attribute not saving in test with FactoryGirl
body: <h2>App</h2>
  
  <p>I have a column <code>bg_image</code> in a table <code>User</code> that uses a carrierwave uploader</p>
  
  <pre><code>mount_uploader :bg_image, UserLogoUploader
  </code></pre>
  
  <p>It gets set in an activeadmin/formtastic form like below:</p>
  
  <pre><code>f.input :bg_image, :label =&gt; "Background Image", :as =&gt; :file
  </code></pre>
  
  <h2>Tests (where the problem is)</h2>
  
  <p>I have a factory for <code>User</code> to make the basic version with all the required fields (which works in other tests), but then when i try to add <code>bg_image</code> it doesn't get saved.</p>
  
  <p>factory:</p>
  
  <pre><code>FactoryGirl.define do
    factory :user do
      stuff "set in factory"
    end
  end
  </code></pre>
  
  <p>test file:</p>
  
  <pre><code>let!(:user) { create(:user, :something =&gt; "that works", :bg_image =&gt; "doesntWork.png") }
  
  it "inspects the element" do
    puts user.inspect
  end
  </code></pre>
  
  <p>prints out</p>
  
  <pre><code>#&lt;User id: 1, stuff: "set in factory", something: "that works", bg_image: nil&gt;
  </code></pre>
  
  <p>I just can't seem to figure out why it doesn't save!</p>
  
  <p>I tried a few things like setting it to <code>File</code> object but that won't work because it needs to be a string</p>