Active storage has_many_attach replaces older images instead of appending them in Rails 6
Was having issues with this but found out behavior was changed from Rails 5. You have to now add the following in your application.rb file:
config.active_storage.replace_on_assign_to_many = false
Notes on this can be found in the change logs here: https://edgeguides.rubyonrails.org/6_0_release_notes.html#active-storage-notable-changes
