关于AFNetWorking 下载进度 显示"The operation couldn’t be completed. Is a directory" 的bug处理

NSURL *URL = [NSURL URLWithString:@"http://cdn.sencha.com/ext/gpl/ext-4.2.1-gpl.zip"];
    NSURLRequest *request = [NSURLRequest requestWithURL:URL];

    //下载请求
    AFHTTPRequestOperation *operation = [[AFHTTPRequestOperation alloc] initWithRequest:request];
    //正确的下载路径 [self getImagePath:@"3.zip"]

    //错误的路径
//    NSArray *path = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,  NSUserDomainMask, YES);
//    NSString *docPath = [path objectAtIndex:0];

    operation.outputStream = [NSOutputStream outputStreamToFileAtPath:[self getImagePath:@"3.zip"] append:YES];
    //下载进度回调
    [operation setDownloadProgressBlock:^(NSUInteger bytesRead, long long totalBytesRead, long long totalBytesExpectedToRead) {
        //下载进度
        float progress = ((float)totalBytesRead) / (totalBytesExpectedToRead);
        NSLog(@"%f",progress);
    }];

    //成功和失败回调
    [operation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
        NSLog(@"ok");
    } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
        NSLog(@"%@",error);
    }];

    [operation start];

<span style="font-family: Arial, Helvetica, sans-serif; background-color: rgb(255, 255, 255);">http://stackoverflow.com/questions/14248757/error-domain-nscocoaerrordomain-code-512-the-operation-couldn-t-be-completed-th</span>