active_model_with_paperclip.rb

module Paperclip
  module Model
    extend ActiveSupport::Concern

    module ClassMethods
      def has_attached_file(name)
        {
          file_name:    :original_filename,
          content_type: :content_type,
          file_size:    :size
        }.each do |(method_name, attachment_method_name)|
          define_method(:"#{name}_#{method_name}") { __send__(name).__send__(attachment_method_name) }
      end
    end
  end
end

class AnyModel
  include ActiveModel::Model
  include Paperclip::Model

  attr_accessor :file
  has_attached_file :file

  {
    presence:     {},
    content_type: {content_type: %w(image/png image/jpeg image/gif)},
    size:         {in: 0..5.megabytes}
  }.each do |(validator, options)|
    validator_class = Paperclip::Validators.const_get("Attachment#{validator.to_s.camelize}Validator")
    validates_with validator_class, {attributes: :file}.merge(options)
  end
end