File is empty. Please upload something more substantial. This error could also be caused by uploads being disabled in your php.ini or by post_max_size being defined as smaller than upload_max_filesize in php.ini.
It sounds like you are either trying to upload a 0kb file or your host doesn't allow uploading through the web server service and maybe only allows it through ftp... Without more info on what you are exactly doing it's hard to say.