title: Carrierwave extension whitelist error message
body: <p>I am using carrierwave with Rails 5. In my extension white list allowed types are jpg, jpeg and png.</p>
  
  <p>The default error message is:</p>
  
  <pre><code>"You are not allowed to upload \"gif\" files, allowed types: jpg, jpeg, png"
  </code></pre>
  
  <p>I don't need customization as message is fine except \"gif\". What I want is:</p>
  
  <pre><code>"You are not allowed to upload gif file, allowed types: jpg, jpeg, png"
  </code></pre>
  
  <p>How to achieve this?</p>