Previewable, first try

active_storage_previewable.rb

module ActiveStoragePreviewable
  extend ActiveSupport::Concern
	
  included do	
    attachments = reflect_on_all_associations.select do |assoc|	
      assoc.klass == ActiveStorage::Attachment
    end
	
    attachments.each do |att|	
      prefix = (/(.*)_attachment\Z/.match att.name.to_s)[1]	
      method_name = att.name.to_s.gsub(/_attachment/, '_shim')	
      define_method(method_name.to_sym) do
        public_send(prefix).attached? ? public_send(prefix) : NullAttachment.new	
      end	
    end
  end	
end

null_attachment.rb

class NullAttachment
  attr_reader :blob
	
  Blob = Struct.new(:content_type)
	
  def initialize(content_type: 'image')
    @blob = Blob.new(content_type)
  end
	
  def variant(resize:)	
    "http://placehold.it/#{resize}"
  end
		
  def preview(resize:)	
    "http://placehold.it/#{resize}"
  end	
end