I tried sending an xlsx file (from path) in the body of a POST http request.
I was able to send the file but it arrived as a corrupted file.(I have no way to try to check what’s the issue on the api, because it’s a api like Amazon etc.)
this is the code I tried:

    public async Task<string> PostPostman()
        {
         try
            {
                string filePath = @"D:example2.xlsx";

                FileStream fs = File.OpenRead(filePath);
                var streamContent = new StreamContent(fs);
                streamContent.Headers.Add("Content-Type", "application/xlsx");
                var client = new RestClient("https://api.xxx.com/v1/FileUpload");
                var request = new RestRequest(Method.Post.ToString());
                request.Method = Method.Post;
                request.AddHeader("Content-Type", "application/xlsx");
                request.AddHeader("Authorization", "Bearer xxxxxxxxxxxxx");
                request.AddParameter("application/xlsx", streamContent.ReadAsStringAsync().Result, ParameterType.RequestBody);
                RestResponse response = client.Execute(request);
                Console.WriteLine(response.Content);
                return response.Content;

            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
                return ex.Message;
            }
        }