title: unable to upload an image using the paperclip gem
body: <p>having installed the latest version of paperclip , i get an error every time i try and upload an image , i can click on the upload button , select my image but when i try to create a listing/image  i get error <strong>Paperclip::Errors::MissingRequiredValidatorError in ListingsController#create</strong> Paperclip::Errors::MissingRequiredValidatorError ..imagemagick installed as well</p>
  
  <p>listings.controller.rb</p>
  
  <pre><code>class ListingsController &lt; ApplicationController
    before_action :set_listing, only: [:show, :edit, :update, :destroy]
  
    # GET /listings
    # GET /listings.json
    def index
      @listings = Listing.all
    end
  
    # GET /listings/1
    # GET /listings/1.json
    def show
    end
  
    # GET /listings/new
    def new
      @listing = Listing.new
    end
  
    # GET /listings/1/edit
    def edit
    end
  
    # POST /listings
    # POST /listings.json
    def create
      @listing = Listing.new(listing_params)  **&lt;------ERROR** 
  
      respond_to do |format|
        if @listing.save
          format.html { redirect_to @listing, notice: 'Listing was successfully created.' }
          format.json { render action: 'show', status: :created, location: @listing }
        else
          format.html { render action: 'new' }
          format.json { render json: @listing.errors, status: :unprocessable_entity }
        end
      end
    end
  
    # PATCH/PUT /listings/1
    # PATCH/PUT /listings/1.json
    def update
      respond_to do |format|
        if @listing.update(listing_params)
          format.html { redirect_to @listing, notice: 'Listing was successfully updated.' }
          format.json { head :no_content }
        else
          format.html { render action: 'edit' }
          format.json { render json: @listing.errors, status: :unprocessable_entity }
        end
      end
    end
  
    # DELETE /listings/1
    # DELETE /listings/1.json
    def destroy
      @listing.destroy
      respond_to do |format|
        format.html { redirect_to listings_url }
        format.json { head :no_content }
      end
    end
  
    private
      # Use callbacks to share common setup or constraints between actions.
      def set_listing
        @listing = Listing.find(params[:id])
      end
  
      # Never trust parameters from the scary internet, only allow the white list through.
      def listing_params
        params.require(:listing).permit(:name, :description, :price, :image)
      end
  end
  </code></pre>
  
  <p>listing.rb</p>
  
  <pre><code>class Listing &lt; ActiveRecord::Base
    has_attached_file :image, :styles =&gt; { :medium =&gt; "200x", :thumb =&gt; "100x100&gt;" }, :default_url =&gt; "missing.jpg"
  end
  </code></pre>
  
  <p>_form.html.erb</p>
  
  <pre><code>&lt;%= form_for(@listing, :html =&gt; { :multipart =&gt; true }) do |f| %&gt;
    &lt;% if @listing.errors.any? %&gt;
      &lt;div id="error_explanation"&gt;
        &lt;h2&gt;&lt;%= pluralize(@listing.errors.count, "error") %&gt; prohibited this listing from being saved:&lt;/h2&gt;
  
        &lt;ul&gt;
        &lt;% @listing.errors.full_messages.each do |msg| %&gt;
          &lt;li&gt;&lt;%= msg %&gt;&lt;/li&gt;
        &lt;% end %&gt;
        &lt;/ul&gt;
      &lt;/div&gt;
    &lt;% end %&gt;
  
    &lt;div class="form-group"&gt;
      &lt;%= f.label :name %&gt;
      &lt;%= f.text_field :name, class: "form-control" %&gt;
    &lt;/div&gt;
    &lt;div class="form-group"&gt;
      &lt;%= f.label :description %&gt;
      &lt;%= f.text_area :description, class: "form-control" %&gt;
    &lt;/div&gt;
   &lt;div class="form-group"&gt;
      &lt;%= f.label :price %&gt;
      &lt;%= f.text_field :price, class: "form-control" %&gt;
    &lt;/div&gt;
    &lt;div class="form-group"&gt;
      &lt;%= f.label :image %&gt;
      &lt;%= f.file_field :image, class: "form-control" %&gt;
    &lt;/div&gt;
     &lt;div class="form-group"&gt;
       &lt;%= f.submit class: "btn btn-primary" %&gt;
     &lt;/div&gt;
  &lt;% end %&gt;
  </code></pre>