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: 
Mimemagic is a dependency of Marcel which is a dependency of a dependency of ... ActiveStorage. The maintainers just changed the license (I think this week), which caused a ruckus.

You want mimemagic at a version between 3.6 and 3.9. You could set it in your Gemfile `gem "mimemagic", "~> 0.3.6"`. Then run `bundle update marcel mimemagic`. If you're on a mac, run `brew install shared-mime-info` first.
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'.