var path = HttpUtility.UrlPathEncode(remotePath.TrimStart('/'));
var url = $"{BASE_ADDRESS}{_cosClientOptions.AppId}/{bucketName}/{path}";
var signature = GenerateSignature(bucketName, remotePath);
var boundary = "---------------" + DateTime.Now.Ticks.ToString("x");
var fileName = Path.GetFileName(remotePath);

var request = new HttpRequestMessage(HttpMethod.Post, url);
request.Headers.Authorization = new AuthenticationHeaderValue("Authorization", signature);
var data = new MultipartFormDataContent(boundary);
data.Add(new ByteArrayContent(Encoding.UTF8.GetBytes("upload")), "op");
data.Add(new StreamContent(uploadStream), "fileContent", fileName);
request.Content = data;

var response = await _httpClient.SendAsync(request);

{"code":-5964,"message":"ERROR_CGI_PARAM_NO_SUCH_OP","request_id":"NWEwMmMwOTVfYzdhMDY4NjRfMjM3OWNfMjQ1OTNj"}