From https://once.com/writebook

app/controllers/action_text/markdown/uploads_controller.rb

@@ -0,0 +1,25 @@
+class ActionText::Markdown::UploadsController < ApplicationController
+  allow_unauthenticated_access only: :show
+
+  before_action do
+    ActiveStorage::Current.url_options = { protocol: request.protocol, host: request.host, port: request.port }
+  end
+
+  def create
+    @record = GlobalID::Locator.locate_signed params[:record_gid]
+
+    @markdown = @record.safe_markdown_attribute params[:attribute_name]
+    @markdown.uploads.attach [ params[:file] ]
+    @markdown.save!
+
+    @upload = @markdown.uploads.attachments.last
+
+    render :create, status: :created, formats: :json
+  end
+
+  def show
+    @attachment = ActiveStorage::Attachment.find_by! slug: "#{params[:slug]}.#{params[:format]}"
+    expires_in 1.year, public: true
+    redirect_to @attachment.url
+  end
+end