title: Undefined method 'reject' when using mongoid-paperclip gem
body: <p>The error I am getting</p>
  
  <pre><code>NoMethodError (undefined method `reject' for #&lt;ActionDispatch::Http::UploadedFile:0x007fab6ea5c370&gt;):
    app/controllers/photos_controller.rb:3:in `create'
  </code></pre>
  
  <p>The form (it's in HAML)</p>
  
  <pre><code>  %form{:id =&gt; 'photo-upload-form', :method =&gt; 'post', :action =&gt; '/photos', :enctype =&gt; 'multipart/form-data'}
      %input{:type =&gt; 'file', :id =&gt; 'photo', :name =&gt; 'photo'}
      %input.btn{:type =&gt; 'submit', :value =&gt; 'Upload Photo'}
  </code></pre>
  
  <p>photos_controller.rb Controller</p>
  
  <pre><code>class PhotosController &lt; ApplicationController
    def create # POST /photos
      photo = Photo.create(params[:photo])
      render :nothing =&gt; true
    end
  end
  </code></pre>
  
  <p>photo.rb Model</p>
  
  <pre><code>class Photo
    include Mongoid::Document
    include Mongoid::Paperclip
  
    has_mongoid_attached_file :photo,
                              :storage =&gt; :s3,
                              :bucket =&gt; '&lt;My_Bucket&gt;',
                              :s3_credentials =&gt; {
                                  :access_key_id =&gt; '&lt;My_Access_Key&gt;',
                                  :secret_access_key =&gt; '&lt;My_Secret_Access_Key&gt;'
                              },
                              :styles =&gt; {
                                  :thumb  =&gt; "100x100",
                                  :medium =&gt; "200x200",
                                  :large =&gt; "600x400"
                              }
  end
  </code></pre>
  
  <p>Gemfile (have run bundle install)</p>
  
  <pre><code>gem 'mongoid-paperclip', :require =&gt; "mongoid_paperclip"
  gem 'aws-sdk'
  </code></pre>
  
  <p>Versions</p>
  
  <pre><code>Ruby 1.9.3
  Rails 3.2.3
  Mongoid 2.4.8
  Mongoid-Paperclip 0.0.7
  Paperclip 3.1.2
  aws-sdk 1.5.7
  </code></pre>
  
  <p>The code is fairly simple and I've followed the docs. No idea what I should do. </p>