title: How to download ActiveStorage attachments to tmpdir
body: <p>The objective is to download an attached file to tempdir for subsequent usage. <a href="http://edgeguides.rubyonrails.org/active_storage_overview.html#downloading-files" rel="noreferrer">The documentation</a> says to use <code>ActiveStorage::Blob#open</code> which seems simple enough.</p>
  
  <p><strong>I'm getting errors so please explain what I'm doing wrong:</strong></p>
  
  <ol>
  <li>Calling <code>@flower.photo.open</code> results in <code>NoMethodError (private method 'open' called for #&lt;ActiveStorage::Attached::One:0x00007f9780625100&gt;)</code></li>
  <li>Calling @flower.photo.blob.open` results in <code>NoMethodError (private method 'open' called for #&lt;ActiveStorage::Blob:0x00007f9780615c50&gt;)</code></li>
  </ol>
  
  <p>Examining <a href="https://github.com/rails/rails/blob/master/activestorage/app/models/active_storage/blob.rb#L169" rel="noreferrer">the source code</a> I'm not sure why I'm getting the private method error.</p>