小程序上传图像失败

小程序端

onChooseAvatar(e) {
    wx.showLoading({ title: '加载中' });
        const that = this; 

        const fs = wx.getFileSystemManager();
        fs.readFile({
          filePath: e.detail.avatarUrl,
          encoding: 'base64',
          success:(res) =>{
            wx.request({
              url: getApp().globalData.host + "User/UploadBase64",
              method: 'POST',
              //header: { 'content-type': 'application/json' },
              data: {
                data: res.data
              },
              success:(r) =>{ 
                this.setData({ avatarUrl: getApp().globalData.url + r.data })
                wx.showToast({ title: '上传成功', icon: 'none' })
             },
              fail(e) { console.error('base64 upload fail', e); }
            });
          },
          fail(e) { console.error('readFile fail', e); }
        });
  }

服务器端

[AllowAnonymous]
[HttpPost, Route("UploadBase64")]
public string UploadBase64(string data)
{
    try
    {
        if (string.IsNullOrEmpty(data)) throw new Exception("no data");
        var bytes = Convert.FromBase64String(data);
        string folder = "/Uploads/image";
        string save = Server.MapPath(folder);
        if (!Directory.Exists(save)) Directory.CreateDirectory(save);
        string file = Guid.NewGuid().ToString() + ".jpg";
        string full = Path.Combine(save, file);
        System.IO.File.WriteAllBytes(full, bytes);
        return Path.Combine(folder, file).Replace('\\', '/');
    }
    catch (Exception ex)
    {
        return ex.Message;
    }
}