title: Active storage has_many_attached is purging previous uploads
body: <p>I'm trying to add more files to a has_many_attached, but when I upload a new file the previous file is purged. Uploading multiple files does add multiple files, but they are all purged on the next upload as well. Is this intended behavior? if so, how do I prevent the purging?</p>
  
  <p>log.rb</p>
  
  <pre><code>class Log &lt; ApplicationRecord
    has_many_attached :uploads
  end
  </code></pre>
  
  <p>_form.html.erb</p>
  
  <pre><code>&lt;%= form_for @log, remote: true do |f| %&gt;
    &lt;%= f.file_field :uploads, multiple: true %&gt;
  &lt;% end %&gt;
  </code></pre>