A private method can't be called with self.

So calling `filename_with_extensions` omitting the second argument like this
8dbc1fbf85/lib/generators/slim/scaffold/scaffold_generator.rb (L10)
causes NoMethodError.

fixes #28275

railties/lib/rails/generators/erb.rb

@@ -17,8 +17,8 @@ module Erb # :nodoc:
           :erb
         end
 
-        def filename_with_extensions(name, format = self.format)
-          [name, format, handler].compact.join(".")
+        def filename_with_extensions(name, file_format = format)
+          [name, file_format, handler].compact.join(".")
         end
     end
   end