title: Error with .webp images when using Active Storage on Heroku
body: <p>When I use Active Storage, and when someone upload a .webp image, and when I run <code>file.attach(io: webp_file, filename: 'file.webp')</code> it works, and then ActiveStorage automatically run a job <code>ActiveStorage::AnalyzeJob</code></p>
  
  <p>But this job raises :</p>
  
  <pre><code>MiniMagick::Error (`identify -format %[orientation] /tmp/ActiveStorage-114989-20180905-4-wak8ob.webp[0]` failed with error:
  identify-im6.q16: delegate failed `'dwebp' -pam '%i' -o '%o'' @ error/delegate.c/InvokeDelegate/1919.
  identify-im6.q16: unable to open file `/tmp/magick-1400SWBHj-p67HrV': No such file or directory @ error/constitute.c/ReadImage/544.
  </code></pre>
  
  <p>Although I am on Heroku-18, and so there is a lib called "libwepb6" (<a href="https://devcenter.heroku.com/articles/stack-packages" rel="noreferrer">https://devcenter.heroku.com/articles/stack-packages</a>) </p>
  
  <p>Do I have to create a Heroku buildpack?</p>