Vanilla Rails isomorphic attachment validations

has_many_attached.rb

# /app/models/concerns/has_many_attached.rb
module HasManyAttached
  extend ActiveSupport::Concern

  class_methods do
    def has_many_attached(name, dependent: :purge_later, service: nil, strict_loading: false, **options)
      super(name, dependent: :purge_later, service: nil, strict_loading: false)
      
      if options[:file_types].any?
        validate "validate_#{name}_file_types".to_sym
        
        singleton_class.define_method("#{name}_file_types") do
          options[:file_types]
        end
        
        define_method("validate_#{name}_file_types") do
          items = public_send(name)
          return unless items.attached?
          
          validations = options[:file_types].map { |type| Regexp.new(type.sub('*', '.*')) }
          
          items.each do |item|
            next if validations.any? { |validation| item.content_type.match?(validation) }
            
            errors.add(name.to_sym, :invalid_type, type: item.content_type)
          end
        end
      end
    end
  end
end

my_model.rb

# /app/models/my_model.rb
class MyModel < ApplicationRecord
  include HasManyAttached
  
  has_many_attached :files,
    dependent: :destroy,
    file_types: ["image/*", "video/*"]
end

my_view.html.erb

<%= form_with(model: my_model) do |form| %>
  <%= form.file_field :files,
        multiple: true,
        accept: form.object.class.files_file_types.join(', ') %>
<% end %>