为啥下载图片总少几个字节

$file_name="maze.png";

if(!file_exists($file_name)){
echo "文件不存在";
return;
}

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

$file_size=filesize($file_name);
//echo $file_size;
header("Content-type: application/octet-stream");
header("Accept-Ranges: bytes");
header("Accept-Length: $file_size");
header("Content-Disposition: attachment; filename=".$file_name);

$buffer=1024;
while(!feof($fp)){
$file_data = fread($fp,$buffer);

echo $file_data;
}



fclose($fp);