Fixes exception in @acts_as_attachable@ when running on Ruby 2.7 (#39862).

Patch by @maeda.

git-svn-id: https://svn.redmine.org/redmine/trunk@22569 e93f8b46-1217-0410-a6f0-8f06a7374b81

lib/plugins/acts_as_attachable/lib/acts_as_attachable.rb

@@ -39,7 +39,7 @@ module Redmine
           end
 
           def param_expression
-            @param_expression ||= Regexp.new("^(#{object_types.join("|")})$")
+            @param_expression ||= Regexp.new("^(#{object_types.to_a.join("|")})$")
           end
         end
       end