title: Store file in directory tmp on heroku Rails
body: <p>In my Delayed Job, I tried to create a file to tmp folder</p>
  
  <pre><code>file_path = Rails.root.join('tmp', "#{file_name}." + file_extension);
  exported_file = kit.to_pdf
  # Save file to disk
  File.open(file_path, 'wb') do |file|
        file &lt;&lt; exported_file 
  end
  </code></pre>
  
  <p>It works well in local but on Heroku there is a error in Delayed Job 
  "No such file  or directory - /app/tmp/test.pdf"</p>
  
  <p>So how I can solve this problem. I do not want to store file in S3.
  Thank you</p>