php下载文件 图片不能打开,该怎么解决

<br /> function fileDown($file_name){<br />        $file_name = iconv("utf-8","gb2312",$file_name);<br />        $file_path = "E:/php/down/".$file_name;<br />        if(!file_exists($file_path)){<br />            echo "文件不存在";<br />            return;<br />        }<br />        $fp = fopen($file_path,"r");<br />        $file_size = filesize($file_path);<br />        Header("Content-type: application/octet-stream");<br />        Header("Accept-Ranges: bytes");<br />        Header("Accept-Length: $file_size");<br />        Header("Content-Disposition: attachment; filename=".$file_name);<br />        $buffer = 1024;<br />        $file_count = 0;<br />        while(!feof($fp) && ($file_size - $file_count>0)){<br />            $file_data = fread($fp,$buffer);<br />            $file_count += $buffer;<br />            echo $file_data;<br />        }<br />        fclose($fp);<br />    }<br />	$file_name="sai.png";<br />	fileDown($file_name);<br />