手机端的代码:
try {
// 实例化URL对象。调用URL有参构造方法，参数是一个url地址；
URL urlObj = new URL(url);
// 调用URL对象的openConnection()方法，创建HttpURLConnection对象；
httpConn = (HttpURLConnection) urlObj.openConnection();
// 调用HttpURLConnection对象setDoOutput(true)、setDoInput(true)、setRequestMethod("POST")；
httpConn.setDoInput(true);
httpConn.setDoOutput(true);
httpConn.setRequestMethod("POST");
// 设置Http请求头信息；（Accept、Connection、Accept-Encoding、Cache-Control、Content-Type、User-Agent）
httpConn.setUseCaches(false);
httpConn.setRequestProperty("Connection", "Keep-Alive");
httpConn.setRequestProperty("Accept", "*/*");
httpConn.setRequestProperty("Accept-Encoding", "gzip, deflate");
httpConn.setRequestProperty("Cache-Control", "no-cache");
httpConn.setRequestProperty("Content-Type",
"multipart/form-data; boundary=" + BOUNDARY);
httpConn.setRequestProperty(
"User-Agent",
"Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30)");
// 调用HttpURLConnection对象的connect()方法，建立与服务器的真实连接；
httpConn.connect();

        // 调用HttpURLConnection对象的getOutputStream()方法构建输出流对象；
        dos = new DataOutputStream(httpConn.getOutputStream());
        // 获取表单中上传控件之外的控件数据，写入到输出流对象（根据HttpWatch提示的流信息拼凑字符串）；
        if (map != null && !map.isEmpty()) {
            for (Map.Entry<String, String> entry : map.entrySet()) {
                String key = entry.getKey();
                String value = map.get(key);
                dos.writeBytes(PREFIX + BOUNDARY + NEWLINE);
                dos.writeBytes("Content-Disposition: form-data; "
                        + "name=\"" + key + "\"" + NEWLINE);
                dos.writeBytes(NEWLINE);
                dos.writeBytes(URLEncoder.encode(value.toString(), charset));
                dos.writeBytes(NEWLINE);
            }
        }

        // 获取表单中上传控件的数据，写入到输出流对象（根据HttpWatch提示的流信息拼凑字符串）；
        if (body_data != null && body_data.length > 0) {
            dos.writeBytes(PREFIX + BOUNDARY + NEWLINE);
            String fileName = filePath.substring(filePath
                    .lastIndexOf(File.separatorChar));
            dos.writeBytes("Content-Disposition: form-data; " + "name=\""
                    + fileType + "\"" + "; filename=\"" + fileName
                    + "\"" + NEWLINE);
            dos.writeBytes(NEWLINE);
            dos.write(body_data);
            dos.writeBytes(NEWLINE);
        }
        dos.writeBytes(PREFIX + BOUNDARY + PREFIX + NEWLINE);
        dos.flush();

服务器端代码：
ioc代码如下：
var ioc = {

    utils : {     
       type : 'cn.guagua.mobile.common.avatarUploadUtils',     
       fields : {         
         sc : {app:'$servlet'}   // 将 ServletContext 对象注入 MyUtils
       }
    },
    tmpFilePool : {     
       type : 'org.nutz.filepool.NutFilePool',     // 临时文件最大个数为 1000 个     
       args : [ '~/opt/uploadlog/avatar/tmps', 1000 ]    
    },
    uploadMediaContext : { 
       type : 'org.nutz.mvc.upload.UploadingContext',     
       singleton : false,     
       args : [ { refer : 'tmpFilePool' } ],
       fields : {
         // 是否忽略空文件, 默认为 false         
         ignoreNull : true, 
         // 单个文件最大尺寸(大约的值，单位为字节，即 1048576 为 1M)         
         maxFileSize : 31457280,
         nameFilter : '^(.+[.])(gif|jpg|png)$' // 正则表达式匹配可以支持的文件名         
       }
    },
    myMediaUpload : {
       type : 'org.nutz.mvc.upload.UploadAdaptor',     
       singleton : false,     
       args : [ { refer : 'uploadMediaContext' }]  
    }

};

@At("/avatarupload")
@AdaptBy(type = UploadAdaptor.class,args = { "ioc:myMediaUpload" })
public Map<?, ?> avatarUpload(HttpServletRequest request,@Param("..")NutMap nm){

    TempFile tf = (TempFile)nm.get("imgBig");
    FieldMeta fm = tf.getMeta();
    String ext_type = fm.getFileExtension();
    String content_type = fm.getContentType();




图片流能正常接收，但是content_type 从手机端传过来是的是nul , 请问这是什么原因?