无法通过Smartsheet SDK C#为SmartsheetClient加载文件

using System;
using System.Net.Http;
using System.Net.Http.Headers;
using Newtonsoft.Json;

public class SmartsheetClient
{
    private const string ApiBaseUrl = "https://api.smartsheet.com/2.0";
    private const string AccessToken = "YOUR_ACCESS_TOKEN";

    public async Task<Sheet> CreateSheet(string sheetName, string filePath)
    {
        using (var client = new HttpClient())
        {
            client.BaseAddress = new Uri(ApiBaseUrl);
            client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", AccessToken);

            var fileContent = File.ReadAllBytes(filePath);
            var base64FileContent = Convert.ToBase64String(fileContent);

            var request = new
            {
                name = sheetName,
                file = base64FileContent
            };

            var jsonRequest = JsonConvert.SerializeObject(request);
            var content = new StringContent(jsonRequest, Encoding.UTF8, "application/json");

            var response = await client.PostAsync("sheets", content);
            response.EnsureSuccessStatusCode();

            var jsonResponse = await response.Content.ReadAsStringAsync();
            var sheet = JsonConvert.DeserializeObject<Sheet>(jsonResponse);

            return sheet;
        }
    }
}

public class Sheet
{
    public long Id { get; set; }
    public string Name { get; set; }
    // Other properties...
}