title: Can't convert Symbol into String with paperclip
body: <p>I'm newbie in ruby on rails, and i'm trying to use paperclip to drag and drop image. When i run my app, i have an error:  </p>
  
  <pre><code> Started POST "/uploads" for 127.0.0.1 at 2014-11-11 08:59:54 +0700
      Processing by UploadsController#create as JSON
        Parameters: {"utf8"=&gt;"✓", "authenticity_token"=&gt;"EXaUGMxPQt6otEUvPAd97IHEq61lYDmmHElxSCpyGp0=", "upload"=&gt;{"image"=&gt;#&lt;ActionDispatch::Http::UploadedFile:0x33f55c8 @original_filename="Chrysanthemum.jpg", @content_type="image/jpeg", @headers="Content-Disposition: form-data; name=\"upload[image]\"; filename=\"Chrysanthemum.jpg\"\r\nContent-Type: image/jpeg\r\n", @tempfile=#&lt;File...AppData/Local/Temp/RackMultipart20141111-1960-9xj2rp&gt;&gt;}}
      Completed 500 Internal Server Error in 2.0ms
  
      TypeError (can't convert Symbol into String):
        app/controllers/uploads_controller.rb:17:in `upload_params'
        app/controllers/uploads_controller.rb:7:in `create
  </code></pre>
  
  <p>Upload_controller:</p>
  
  <pre><code>class UploadsController &lt; ApplicationController
    def new
      @upload = Upload.new
    end
  
    def create
      @upload = Upload.create(upload_params)
      if @upload.save
        render json: { message: "success" }, :status =&gt; 200
      else
        render json: { error: @upload.errors.full_messages.join(',')}, :status =&gt; 400
      end
    end
  
     private
    def upload_params
      params.require(:upload).permit(:image)
    end
  end
  </code></pre>
  
  <p>Upload_model:</p>
  
  <pre><code>class Upload &lt; ActiveRecord::Base
    attr_accessible :image
    has_attached_file :image, :styles =&gt; { :medium =&gt; "300x300&gt;",:thumb =&gt; "100x100&gt;" }
  
    validates_attachment  :image, 
          :presence =&gt; true,
          :content_type =&gt; { :content_type =&gt; /\Aimage\/.*\Z/ },
          :size =&gt; { :less_than =&gt; 1.megabyte }
  end
  </code></pre>
  
  <p>upload.html</p>
  
  <pre><code>&lt;%= form_for(@upload, html: { multipart: true, class: "dropzone"}) do |f|  %&gt;
          &lt;div class="fallback"&gt;
              &lt;%= f.file_field :image %&gt;
              &lt;br&gt;
              &lt;%= f.submit "Upload" %&gt;
          &lt;/div&gt;
          &lt;% end %&gt;
  </code></pre>
  
  <p>Maybe, the problem is i don't use "strong_parameters" but when i add gem 'strong_parameters', it doesn't work. I don't know how to fix that :( Help me!</p>