在 php 中通过 CURL 上传文件

$file = realpath('gif/1.gif'); //要上传的文件
$fields['f'] = '@'.$file;
$ch = curl_init();  
curl_setopt($ch, CURLOPT_URL,"http://localhost/ajax_server.php?id=1");  
curl_setopt($ch, CURLOPT_POST, 1 );
curl_setopt($ch, CURLOPT_POSTFIELDS, $fields);
curl_exec ($ch);  
curl_close ($ch);

$contents =<<< 'TEXT'
数据报中应该是
Content-Disposition: form-data; name="userfile"; filename="file_name"
Content-Type: 文档类型
文件内容
这样的格式，我只实现了文件名部分，文档类型不知道如何实现。
这样上传后就取不到 type 的值
curl_upload_server.php
<xmp>
<?php
print_r($_FILES);
echo "文件内容:\n";
$p = current($_FILES);
readfile($p['tmp_name']);
TEXT;
$fields['f"; filename="x.x'] = $contents; //这个关联键的写法很怪异吧？
$ch = curl_init();  
curl_setopt($ch, CURLOPT_URL,"http://localhost/curl_upload_server.php");  
curl_setopt($ch, CURLOPT_POST, 1 );
curl_setopt($ch, CURLOPT_POSTFIELDS, $fields);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$s = curl_exec ($ch);  
curl_close ($ch);  
echo $s;

$contents =<<< 'TEXT'
上传文件在数据报中应该是
Content-Disposition: form-data; name="userfile"; filename="file_name"
Content-Type: 文档类型
文件内容
这样的格式
以下是服务器端代码
curl_upload_server.php
<xmp>
<?php
print_r($_FILES); //检查上传信息
echo "文件内容:\n";
$p = current($_FILES);
readfile($p['tmp_name']);  //输出上传的文件
TEXT;
$varname = 'my';
$name = '3.txt';
$type = 'text/plain';
$key = "$varname\"; filename=\"$name\r\nContent-Type: $type\r\n";
$fields[$key] = $contents;
$ch = curl_init();  
curl_setopt($ch, CURLOPT_URL,"http://localhost/curl_upload_server.php");  
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST" );
curl_setopt($ch, CURLOPT_POSTFIELDS, $fields);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$s = curl_exec ($ch);  
curl_close ($ch);  
echo $s;