Question: I'm getting an error when uploading a release to heroku.

! LoadError: cannot load such file -- mimemagic/overlay

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 (<https://github.com/rails/rails/issues/41757>) where them gem has been amended in the last few days and wonder whether I've been caught by a breaking update.

Answer: 
in my case, just update rails version to

```
gem "rails", "~> 5.2.5"

```

then *bundle update*
In the end is was quite simple.
Include:-
gem 'mimemagic', '~> 0.3.10'
in the Gemfile. The entry specifically requires a set version, thus avoiding the versions that got pulled. At some point in the future I'll replace 'carrierwave' with the vanilla 'ActionStorage'.