Fixed: error when downloading a file with no matching mime type (#3782).

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@3281 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/controllers/attachments_controller.rb

@@ -82,6 +82,6 @@ private
     if content_type.blank?
       content_type = Redmine::MimeType.of(attachment.filename)
     end
-    content_type
+    content_type.to_s
   end
 end