title: Rails with Paperclip ignore empty attachments
body: <p>I am using paperclip with rails and it works fine, however my problem is when updating if the i don't reselect the image, then it saves the record as nil. Here is my code:</p>
  
  <pre><code>&lt;div class="form-group"&gt;
    &lt;div class="media"&gt;
      &lt;%= image_tag program_avatar(b), :id =&gt; 'avatar', :class =&gt; 'thumbnail media-object pull-left', :height =&gt; 100, :width =&gt; 100 %&gt;
      &lt;div class="media-body padding-top-40"&gt;
        &lt;%= b.file_field :avatar, :class =&gt; 'file-upload' %&gt;
      &lt;/div&gt;
    &lt;/div&gt;
  &lt;/div&gt;
  
  &lt;div class="form-group"&gt;
    &lt;div class="media"&gt;
      &lt;%= image_tag program_banner(b), :class =&gt; 'thumbnail media-object pull-left', :height =&gt; 100, :width =&gt; 300 %&gt;
      &lt;div class="media-body padding-top-40"&gt;
        &lt;%= b.file_field :banner, :class =&gt; 'file-upload' %&gt;
      &lt;/div&gt;
    &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>and the controller:</p>
  
  <pre><code>respond_to do |format|
    if @program.update(program_params)
      format.html { redirect_to(program_path(@program), :notice =&gt; "Program updated") }
      format.js { render :json =&gt; @program.json }
    else
      format.html { render :new, :notice =&gt; "Error please try again" }
      format.js { render :json =&gt; "Error please try again" }
    end
  end
  </code></pre>
  
  <p>The models:</p>
  
  <p><em>Book model</em>:</p>
  
  <pre><code>has_one :book_content, :dependent =&gt; :destroy  
  accepts_nested_attributes_for :book_content, :allow_destroy =&gt; true
  </code></pre>
  
  <p>Book_content model:</p>
  
  <pre><code>has_attached_file :avatar, :styles =&gt; { :medium =&gt; "300x300&gt;", :thumb =&gt; "100x100&gt;" }, :default_url =&gt; "/images/:style/missing.png"
  has_attached_file :banner, :styles =&gt; { :header =&gt; "600x150", :setting =&gt; "300x100"  }, :default_url =&gt; "/images/:style/found.jpeg"
  belongs_to :book
  </code></pre>
  
  <p>this is a nested form, but because it has multiple attachments so i cannot use <code>reject_if</code></p>
  
  <p>How can tell paperclip to keep the original files if no file is selected?</p>
  
  <p>Thank you</p>