app/validators/image_validator.rb

class ImageValidator < ActiveModel::EachValidator
  IMAGE_TYPES_ALLOWED = %i[jpeg png gif].freeze
  IMAGE_MAX_WIDTH = 4096
  IMAGE_MAX_HEIGHT = 4096

  def validate_each(record, attribute, value)
    return if value.blank?

    image = FastImage.new(value)

    return if image_type_valid?(image) && pixel_dimensions_valid?(image)

    record.errors.add(
      attribute,
      options[:message] ||
        'must be a JPEG, PNG, or GIF, less than 4096 pixels wide or high'
    )
  end

  def image_type_valid?(image)
    image.type && IMAGE_TYPES_ALLOWED.include?(image.type)
  end

  def pixel_dimensions_valid?(image)
    image.size && image.size[0] <= IMAGE_MAX_WIDTH &&
      image.size[1] <= IMAGE_MAX_HEIGHT
  end
end