title: Custom form_for field accessible in model
body: <p>I have custom field in <code>form_for</code></p>
  
  <pre><code>check_box_tag "files_to_delete[]", :attach.id
  </code></pre>
  
  <p>Permitted params:</p>
  
  <pre><code>params.require(:post).permit( ... , :files_to_delete =&gt; [])
  </code></pre>
  
  <p>I have <code>before_action :some_method</code> in my model, in which i want to have access to the <code>files_to_delete</code>:</p>
  
  <pre><code>def some_method
    files_to_delete.each do |attach|
      attach.clear
    end
  end
  </code></pre>
  
  <p>But i get:</p>
  
  <pre><code>undefined local variable or method `files_to_delete' for #&lt;Post:0x007f5c4cb51ad0&gt;
  </code></pre>