title: Download an active Storage attachment to disc
body: <p><a href="http://edgeguides.rubyonrails.org/active_storage_overview.html#downloading-files" rel="noreferrer">The guide</a> says that I can save an attachment to disc to run a process on it like this:</p>
  
  <pre><code>message.video.open do |file|
    system '/path/to/virus/scanner', file.path
    # ...
  end
  </code></pre>
  
  <p>My model has an attachment defined as:</p>
  
  <pre><code>has_one_attached :zip
  </code></pre>
  
  <p>And then in the model I have defined: </p>
  
  <pre><code>def process_zip      
    zip.open do |file|
      # process the zip file
    end
  end
  </code></pre>
  
  <p>However I am getting an error :</p>
  
  <pre><code>private method `open' called
  </code></pre>
  
  <p>on the zip.open call.  </p>
  
  <p>How can I save the zip locally for processing?</p>