title: Heroku: LoadError: cannot load such file -- mimemagic/overlay
body: <p>I'm getting an error when uploading a release to heroku.</p>
  <p>!     LoadError: cannot load such file -- mimemagic/overlay</p>
  <p>I successfully ran the 'bundle exec rake -P' command as advised but could not see any mimemagic in the output. I've never had to think about mimemagic before. I think its pulled in by other gems, notably 'carrierwave'. I've found a conversation (<a href="https://github.com/rails/rails/issues/41757" rel="nofollow noreferrer">https://github.com/rails/rails/issues/41757</a>) where them gem has been amended in the last few days and wonder whether I've been caught by a breaking update.</p>