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: 
**What worked** was updating rails in my gemfile to `gem 'rails', '~> 5.2.5'` and *then* running `bundle update` as @asharijuang mentioned above.

Then I was able to finally push to Heroku after being stuck on this for a few days

A few of the things I tried unsuccessfully:

`gem install mimemagic`

`brew install shared-mime-info` *then* `bundle update marcel mimemagic`
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'.