Special Mail Tags not working

Hello,

I am trying to send User’s name in the Mail title by using a special tag [_user_first_name] something like this in the mail title and also Message body. But it comes as blank. I am using this on a document upload form so that I will know via email, who uploaded the document. The user is logged in while uploading. Please let me know any workaround. I am completely new to wordpress. Any help is appreciated.

Thanks.