I have a created a server code which accepts file through post command.
Server side code:

   app.MapPost("/upload2", async (IFormFile file) =>
   {
    List<string> validextensions = new List<string>() { ".xlsx" };
    string extension = Path.GetExtension(file.FileName);
    if (!validextensions.Contains(extension))
    {
        return $"Extention is not valid({string.Join(",", validextensions)})";
    }
    string filename = "File1" + extension;
    string path = @"D:/";
    using FileStream stream = new FileStream(Path.Combine(path, filename), FileMode.Create);
    file.CopyToAsync(stream);

    return filename;
   });

I believe this code is ok because it works fine in postman. However, I failed sending file through C#!

Client code:

public async Task SendExcelFileToServer(string filepath, string filename, string InnerAddress)
{
    try
    {
        // Create an instance of HttpClient
        using (var httpClient = new HttpClient())
        {
            // Set the request URI
            string requestUri = ServerAddressTextBox.Text.Trim() + InnerAddress;

            // Create multipart form data content
            using (var form = new MultipartFormDataContent())
            {
                string antiforgeryToken, cookie;
                (antiforgeryToken, cookie) = await GetAntiforgeryToken();

                // Add file part
                using (var fileContent = new ByteArrayContent(File.ReadAllBytes(filepath)))
                {
                    //form.Headers.ContentType = MediaTypeHeaderValue.Parse("application/vnd.openxmlformats-officedocument.spreadsheetml.sheet");
                    //form.Add(fileContent, "file", filename);

                    // This Is obligatory inn .net8! :)
                    // Add the RequestVerificationToken header

                    form.Headers.Add("RequestVerificationToken", "CfDJ8IobTqaCf0xCi3hgie2deVpx3DCZeGmVD28L6YAsM51Szh1AF0afHexqh7O768rwdYqOqLx8ph3yM86l7bg_phwtnHFspWVdFBtKScLlooFE_10wA6HidiiMZT6TbqZsR5Q1l6C7qJ9Zcuj_1bhxt10");

                    // Add the cookie header
                    form.Headers.Add("Cookie", ".AspNetCore.Antiforgery.p8-WnM9DThg=CfDJ8IobTqaCf0xCi3hgie2deVrgEG5nQyY-SHCgXvPCDh8LFzaqG7fOX41qgeGCXUcjPKgAPvPRqEUgYhzhHLTfcnClNq5rmQIZ-CkycC_V0vzBYhSgXdCVnL_9_ftvHXhYl7S3RR575hPwsAUvKg9D8kQ");
                    //form.Headers.ContentType = MediaTypeHeaderValue.Parse("application/octet-stream");
                    //httpClient.DefaultRequestHeaders.Add("Accept", "*/*");

                    //httpClient.DefaultRequestHeaders.Add("Accept-Encoding", "gzip, deflate, br");
                    //httpClient.DefaultRequestHeaders.Add("Connection", "keep-alive");

                    // Send post request
                    HttpResponseMessage response = await httpClient.PostAsync(requestUri, form);

                    // Check response
                    response.EnsureSuccessStatusCode();
                }




            }
        }
    }
    catch
    {
        MessageBox.Show("Try sending data again and check your connection!");
    }
}

I added
RequestVerificationToken to my postman post command manually. Also, I added it to my post command in C#.

As you can see based on my comments in client code, I have tried so many options. But, they all failed.