用nsurlconnection上传图片出错，求助

直接上代码

     NSURL *url = [NSURL URLWithString:urlStr];
//    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    NSMutableURLRequest *request = [[NSMutableURLRequest alloc]init ];   
    NSString *boundary = @"---------------------------14737809831466499882746641449";
    NSMutableData *postData = [[NSMutableData alloc] init];
   [postData appendData:[[NSString stringWithFormat:@"\r\n--%@\r\n",boundary] dataUsingEncoding:NSUTF8StringEncoding]];
    [postData appendData:[[NSString stringWithFormat:@"Content-Disposition:form-data;name=\"userfile\";filename=\"%@\"\r\n",fileName]
                          dataUsingEncoding:NSUTF8StringEncoding]];
//    [postData appendData:[@"Content-Type:application/octet-stream\r\n\r\n" dataUsingEncoding:NSUTF8StringEncoding]];
    [postData appendData:[@"Content-Type:image/png\r\n\r\n" dataUsingEncoding:NSUTF8StringEncoding]];
    [postData appendData:[NSData dataWithData:imgData]];
    [postData appendData:[[NSString stringWithFormat:@"\r\n--%@--\r\n",boundary] dataUsingEncoding:NSUTF8StringEncoding]];
//    NSLog(@"data:%@",postData);
     [request setHTTPBody:postData];
    [request addValue:[NSString stringWithFormat:@"%d",(int)[postData length]] forHTTPHeaderField:@"Content-Length"];
    request.URL = url;
    [request setHTTPMethod:@"POST"];    
    NSString *contentType = [NSString stringWithFormat:@"multipart/form-data;boundary=%@",boundary];
    [request addValue:contentType forHTTPHeaderField:@"Content-Type"];  
     NSOperationQueue *queue = [[NSOperationQueue alloc]init];
     [NSURLConnection sendAsynchronousRequest:request queue:queue completionHandler:^(NSURLResponse *response, NSData *data, NSError *connectionError) {
         NSString *html = [[NSString alloc]initWithData:data encoding:NSUTF8StringEncoding];
         NSLog(@"html:%@",html);
         NSError *error = nil;
         id jsonObj = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingAllowFragments error:&error];
         if (jsonObj != nil) {
             NSLog(@"json:%@",jsonObj);
         }
         if (error != nil) {
             NSLog(@"err:%@",error);
         }

    }];

结果总是出错，请问我的代码有什么问题吗？