DialogFilter was inert before 5.3.2 - it stored to ViewState and was never read, so a page restricting uploads to images accepted anything, including executables. It now feeds the allowed-extension list, with ValidateOption still winning where both are set.

The two message properties above are also new in 5.3.2. v4 rendered these strings itself; in v5 the text came from the engine's locale packs and the properties were never read, so a translated site silently reverted to English. v4's positional placeholders ({0}, {1}, {2}) are translated to the engine's named tokens for you.