Hi Ken,

Thanks you for  the quick reply.

I admit that I only tried upload file with .msi extension and for that one still get message "This file extension is not allowed! File extensions allowed: 'null'". 

After your reply I tried other extensions and usually works fine. But if I want upload .xlsx file, the error message show again. Then I set  AllowedFileExtensions = "xlsx" and the message still show again (This file extension is not allowed! File extensions allowed: 'xlsx').  

Usually works fine, but not for all extensions. 

(I am using AjaxUploader on ASP.NET MVC platform)

Regards,

Radomir