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//设置HTTP Content-Length
    NSString *lenStr = [NSString stringWithFormat:@"%ld",request.HTTPBody.length];
    [request setValue:lenStr forHTTPHeaderField:@"Content-Length"];
    
    //设置HTTP Content-Type
    [request setValue:formData.contentType forHTTPHeaderField:@"Content-Type"];
    
    // 5.发送请求
    [NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *response, NSData *data, NSError *connectionError) {
       //处理请求结果
    }];

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原文：http://451143112.blog.51cto.com/10235476/1651780