Issue
A Ruby App using mimemagic is having difficulties building on Heroku.
Resolution
There have been reports of build failures for Ruby apps using mimemagic due to a recent yank of mimemagic in relation to licensing.
This issue is outside of Heroku Platforms control, as mimemagic is third party. However, mimemagic has been updated and is now using an MIT license, and as such you will be able to use mimemagic by updating the version to one using the new license. The easiest way to resolve this is to update Ruby itself to remove the dependency to mimemagic from rails/activestorage and use Marcel instead.
Further information found in the following:
https://github.com/rails/rails/issues/41757#issuecomment-808445077
https://weblog.rubyonrails.org/2021/3/26/marcel-upgrade-releases/
https://github.com/mimemagicrb/mimemagic/blob/HEAD/CHANGELOG.md#039-2021-03-25