title: Issues with validation using carrierwave gem in rails 3
body: <p>Hey so this is really bugging me. I've implemented Carrierwave into my app with a simple image upload. Im trying to validate the image to be the correct filetype and I'm getting a few strange results. Heres the relevant code:</p>
  
  <pre><code>class Art &lt; ActiveRecord::Base
  belongs_to :collection
  attr_accessible :image, :collection_id, :remote_image_url
  mount_uploader :image, ImageUploader
  validates_integrity_of :image
  validates_presence_of :image
  validates_processing_of :image
  end
  </code></pre>
  
  <p><code> </p>
  
  <pre><code>class ImageUploader &lt; CarrierWave::Uploader::Base
  
  include CarrierWave::MiniMagick
  
  storage :file
  
  def store_dir
    "uploads/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
  end
  
  def extension_white_list
    %w(jpg jpeg gif)
  end
  </code></pre>
  
  <p><code></p>
  
  <pre>class ArtsController &lt; ApplicationController
  
  def new
    @art = Art.new(:collection_id =&gt; params[:collection_id])
  end
  
  def edit
    @art = Art.find(params[:id])
  end
  
  def create
    @art = Art.new(params[:art])
      if @art.save
        flash[:notice] = "Successfully created art."
    redirect_to @art.collection
      else
        flash[:error] = @art.errors
    render :action =&gt; :new
     end
  end
  
  def update
    @art = Art.find(params[:id])
  
      if @art.update_attributes(params[:art])
    flash[:notice] = "Successfully updated art."
    redirect_to @art.collection
      else
    flash[:error] = @art.errors
    render :action =&gt; :edit
    end
  end
  
  def destroy
    @art = Art.find(params[:id])
    @art.destroy
  flash[:notice] = "Successfully destroyed art."
  redirect_to @art.collection
    end
  end
  </code></pre>
  
  <p> </p>
  
  <pre>en:
    errors:
     messages:
       carrierwave_processing_error: "Cannot resize image."
       carrierwave_integrity_error: "Not an image."
       carrierwave_download_error: "Couldn't download image."
       extension_white_list_error: "NOT ALLOWED"
       extension_black_list_error: "You are not allowed to upload %{extension} files, 
       prohibited types: %{prohibited_types}"
  </code></pre>
  
  <p>So when i create a new piece of art with a png file format an error will occur that the image cannot be blank. I understand this is the validates_presence_of being trigger but i am supplying it with something just not the correct format. If i remove the validates_presence_of non correct file formats won't display but will still be created. I also assed the i18n error messages to my en.yml file. I'm somewhat new to rails so i assume there is an issue in my controller with creating each piece of art but im not sure what I'm doing wrong.</p>