rails-docstore/app/models/page_preview.rb

class PagePreview < ApplicationRecord
  belongs_to :content

  has_attached_file :preview,
  {
    adapter_options: { hash_digest: Digest::SHA256 },
    url: "/system/:class/:attachment/:id_partition/:style/:hash.:extension",
    hash_secret: Rails.application.secrets.secret_key_base
  }
  validates_attachment_content_type :preview, content_type: 'image/png'
end