From https://once.com/writebook

lib/rails_ext/action_text_markdown.rb

@@ -0,0 +1,42 @@
+module ActionText
+  class Markdown < Record
+    DEFAULT_RENDERER_OPTIONS = {
+      filter_html: false
+    }
+
+    DEFAULT_MARKDOWN_EXTENSIONS = {
+      autolink: true,
+      highlight: true,
+      no_intra_emphasis: true,
+      fenced_code_blocks: true,
+      lax_spacing: true,
+      strikethrough: true,
+      tables: true
+    }
+
+    mattr_accessor :renderer, default: Redcarpet::Markdown.new(
+      Redcarpet::Render::HTML.new(DEFAULT_RENDERER_OPTIONS), DEFAULT_MARKDOWN_EXTENSIONS)
+
+    belongs_to :record, polymorphic: true, touch: true
+
+    def to_html
+      (renderer.try(:call) || renderer).render(content).html_safe
+    end
+  end
+end
+
+module ActionText::Markdown::Uploads
+  extend ActiveSupport::Concern
+
+  included do
+    has_many_attached :uploads, dependent: :destroy
+  end
+end
+
+ActiveSupport.on_load :active_storage_attachment do
+  class ActionText::Markdown
+    include ActionText::Markdown::Uploads
+  end
+end
+
+ActiveSupport.run_load_hooks :action_text_markdown, ActionText::Markdown