Because it throwing an error in Heroku during deploy:
Fetching gem metadata from https://rubygems.org/........
Your bundle is locked to mimemagic (0.3.3), but that version could not be found
in any of the sources listed in your Gemfile. If you haven't changed sources,
that means the author of mimemagic (0.3.3) has removed it. You'll need to update
your bundle to a version other than mimemagic (0.3.3) that hasn't been removed
in order to install.
Update the mimemagic to latest version - 0.3.10
Because it throwing an error in Heroku during deploy: Fetching gem metadata from https://rubygems.org/........ Your bundle is locked to mimemagic (0.3.3), but that version could not be found in any of the sources listed in your Gemfile. If you haven't changed sources, that means the author of mimemagic (0.3.3) has removed it. You'll need to update your bundle to a version other than mimemagic (0.3.3) that hasn't been removed in order to install.