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)

    Zoho Developer Community

                              Zoho Desk Resources

                              • Desk Community Learning Series

                              • Digest

                              • Functions

                              • Meetups

                              • Kbase

                              • Resources

                              • Glossary

                              • Desk Marketplace

                              • MVP Corner

                              • Word of the Day

                                  Zoho Marketing Automation

                                          Manage your brands on social media

                                                Zoho TeamInbox Resources

                                                  Zoho DataPrep Resources

                                                    Zoho CRM Plus Resources

                                                      Zoho Books Resources

                                                        Zoho Subscriptions Resources

                                                          Zoho Projects Resources

                                                            Zoho Sprints Resources

                                                              Qntrl Resources

                                                                Zoho Creator Resources

                                                                  Zoho WorkDrive Resources

                                                                    Zoho Campaigns Resources

                                                                      Zoho CRM Resources

                                                                      • CRM Community Learning Series

                                                                        CRM Community Learning Series

                                                                      • Tips


                                                                      • Functions


                                                                      • Meetups


                                                                      • Kbase


                                                                      • Resources


                                                                      • Digest


                                                                      • CRM Marketplace

                                                                        CRM Marketplace

                                                                      • MVP Corner

                                                                        MVP Corner

                                                                          Design. Discuss. Deliver.

                                                                          Create visually engaging stories with Zoho Show.

                                                                          Get Started Now