php下载文件图片不能打开,该怎么解决

 function fileDown($file_name){

        $file_name = iconv("utf-8","gb2312",$file_name);

        $file_path = "E:/php/down/".$file_name;

        if(!file_exists($file_path)){

            echo "文件不存在";

            return;

        }

        $fp = fopen($file_path,"r");

        $file_size = filesize($file_path);

        Header("Content-type: application/octet-stream");

        Header("Accept-Ranges: bytes");

        Header("Accept-Length: $file_size");

        Header("Content-Disposition: attachment; filename=".$file_name);

        $buffer = 1024;

        $file_count = 0;

        while(!feof($fp) && ($file_size - $file_count>0)){

            $file_data = fread($fp,$buffer);

            $file_count += $buffer;

            echo $file_data;

        }

        fclose($fp);

    }

	$file_name="sai.png";

	fileDown($file_name);