Correctly generate application_mailer.rb in mountable engines

- Followup of https://github.com/rails/rails/pull/24161.

actionmailer/lib/rails/generators/mailer/mailer_generator.rb

@@ -11,8 +11,8 @@ def create_mailer_file
         template "mailer.rb", File.join('app/mailers', class_path, "#{file_name}_mailer.rb")
 
         in_root do
-          if self.behavior == :invoke && !File.exist?('app/mailers/application_mailer.rb')
-            template 'application_mailer.rb', 'app/mailers/application_mailer.rb'
+          if self.behavior == :invoke && !File.exist?(application_mailer_file_name)
+            template 'application_mailer.rb', application_mailer_file_name
           end
         end
       end
@@ -23,6 +23,15 @@ def create_mailer_file
         def file_name
           @_file_name ||= super.gsub(/_mailer/i, '')
         end
+
+      private
+        def application_mailer_file_name
+          @_application_mailer_file_name ||= if mountable_engine?
+                                             "app/mailers/#{namespaced_path}/application_mailer.rb"
+                                           else
+                                             "app/mailers/application_mailer.rb"
+                                           end
+        end
     end
   end
 end

actionmailer/lib/rails/generators/mailer/templates/application_mailer.rb

+<% module_namespacing do %>
 class ApplicationMailer < ActionMailer::Base
   default from: 'from@example.com'
   layout :mailer
 end
+<% end %>

railties/test/generators/plugin_generator_test.rb

@@ -656,6 +656,19 @@ def test_generate_application_record_when_does_not_exist_in_mountable_engine
     end
   end
 
+  def test_generate_application_mailer_when_does_not_exist_in_mountable_engine
+    run_generator [destination_root, '--mountable']
+    FileUtils.rm "#{destination_root}/app/mailers/bukkits/application_mailer.rb"
+    capture(:stdout) do
+      `#{destination_root}/bin/rails g mailer User`
+    end
+
+    assert_file "#{destination_root}/app/mailers/bukkits/application_mailer.rb" do |mailer|
+      assert_match(/module Bukkits/, mailer)
+      assert_match(/class ApplicationMailer < ActionMailer::Base/, mailer)
+    end
+  end
+
   def test_after_bundle_callback
     path = 'http://example.org/rails_template'
     template = %{ after_bundle { run 'echo ran after_bundle' } }