Upload json File to Zoho Docs with C# - "EMPTY_OR_NO_FILE_SPECIFIED"

Upload json File to Zoho Docs with C# - "EMPTY_OR_NO_FILE_SPECIFIED"

For my business work, I'm trying to write a method in C# to upload json files to a contractor zoho docs account.
We have tried some different ways but with no success.
Connection is correctly established , if I insert a wrong token or folderId I got a bad request response.
But I didn't succeed to correctly insert the content of the file.
Always received response message  : "EMPTY_OR_NO_FILE_SPECIFIED"
Could someone help on this ?
Here is one of the test methods that return this response message : 
        public static void PostStockUnal(int sourceApiId)
            string accessToken = "Zoho-oauthtoken " + token // generated with other method

            HttpContent content = new StringContent(stockReport, Encoding.UTF8, "application/json");
            Console.WriteLine("Start Request");

            HttpClient client = new HttpClient();
            client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("multipart/form-data"));
            client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", accessToken);

            Console.WriteLine("Content = " + content.Headers);

            var x = content.ReadAsStringAsync().Result;
            Console.WriteLine("Content = " + x);
            var resp = client.PostAsync("https://apidocs.zoho.com/files/v1/upload" + "?filename=Stock_snapshot20210210.json&fid=lz6x5c7bb900ad31343eeb9fe1a76c63d2a52"
                            , content).Result;
            Console.WriteLine("Resp Status Code= " + resp.StatusCode);
            Console.WriteLine("Res Read=" + resp.Content.ReadAsStringAsync().Result);
            if (resp.IsSuccessStatusCode)

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