背景：使用multipart/form-data格式提交图片或者文件到服务器。

方法：使用http请求POST方式，然后在请求头里设置Content-Type为multipart/from-data指定请求的格式。

参数格式：

普通参数：
--boundry\r\n
Content-Disposition: form-data; name=\"%@\"\r\n           
\r\n
value
\r\n

文件参数：
--boundry\r\n
Content-Disposition:form-data; name=\"%@\"; filename=\"%@\"\r\nContent-Type: %@\r\n         
\r\n
data
\r\n

参数结尾：
--boundry--\r\n

例子：

服务器要求传参：{''upload'': imgdata, "uid": uid}

#pragma mark ------ 上传头像 POST /file/uploadHead
- (void)uploadHeadImage:(UIImage *)image usrId:(NSString *)usrId onSuccess:(void (^)(NSDictionary *dict))onSuccess onFailure:(void(^)(NSError *error))onFailure {

//BaseWebAddr：服务器地址
    NSString *urlStr = [BaseWebAddr stringByAppendingString:@"file/uploadHead"];
    NSURL *url = [NSURL URLWithString:urlStr];
  
    //在此声明一个字符串常量,代替下面的 BOUNDARY 字符串
//   NSString * const POST_BOUNDS = @"----iOSKitFormBoundarycIO0B6mmsXU6koBg";
    
    NSData *imageData;
    NSString *imageFormat;
    if (UIImagePNGRepresentation(image) != nil) {
        imageFormat = @"Content-Type: image/png \r\n";
        imageData = UIImagePNGRepresentation(image);
        
    }else{
        imageFormat = @"Content-Type: image/jpeg \r\n";
        imageData = UIImageJPEGRepresentation(image, 1.0);
        
    }
    
    NSMutableURLRequest *request  = [NSMutableURLRequest requestWithURL:url];
    request.HTTPMethod = @"POST";
    
    //设置请求实体
    NSMutableData *body = [NSMutableData data];
    ///文件参数
    [body appendData:[self getDataWithString:@"--BOUNDARY\r\n" ]];
    NSString *disposition = [NSString stringWithFormat:@"Content-Disposition: form-data; name=\"%@\"; filename=\"%@.jpg\"\r\n",@"upload",usrId];
    [body appendData:[self getDataWithString:disposition ]];
    [body appendData:[self getDataWithString:imageFormat]];
    [body appendData:[self getDataWithString:@"\r\n"]];
    [body appendData:imageData];
    [body appendData:[self getDataWithString:@"\r\n"]];
    
    //普通参数
    [body appendData:[self getDataWithString:@"--BOUNDARY\r\n" ]];
    //上传参数需要key
    NSString *dispositions = [NSString stringWithFormat:@"Content-Disposition: form-data; name=\"%@\"\r\n",@"uid"];
    [body appendData:[self getDataWithString:dispositions ]];
    [body appendData:[self getDataWithString:@"\r\n"]];
    [body appendData:[self getDataWithString:usrId]];
    [body appendData:[self getDataWithString:@"\r\n"]];

    //参数结束
    [body appendData:[self getDataWithString:@"--BOUNDARY--\r\n"]];
    request.HTTPBody = body;
    
    //设置请求体长度
    NSInteger length = [body length];
    [request setValue:[NSString stringWithFormat:@"%ld",length] forHTTPHeaderField:@"Content-Length"];
    //设置 POST请求文件上传
    [request setValue:@"multipart/form-data; boundary=BOUNDARY" forHTTPHeaderField:@"Content-Type"];
    
    NSURLSessionDataTask *dataTask = [self.session dataTaskWithRequest:request completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
        
        NSJSONSerialization *object = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingMutableLeaves error:nil];
        NSDictionary *dict = (NSDictionary *)object;
        NSLog(@"=====%@",[dict objectForKey:@"success"]);
    }];
    //开始任务
    [dataTask resume];
    
}

-(NSData *)getDataWithString:(NSString *)string{
    
    NSData *data = [string dataUsingEncoding:NSUTF8StringEncoding];
    
    return data;
    
}