Since we're going to remove Paperclip and Active Storage doesn't provide
any validations, we have to either write our own validation rules or use
a different gem.
We're using the file_validators gem instead of the
`active_storage_validations` gem because the latter doesn't support
proc/lambda objects in size and content type definitions. We need to use
them because in our case these values depend on settings stored in the
database.
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