您的问题是您使用的是异步函数，而不是 await .
您使用的是ASP.NET Core，因此您应该 (读为"must")必须使用异步所有模式:

Your problem is that you are using an asynchronous function and not awaiting it.
You are using ASP.NET Core so you should (read "must") use the async-all-the-way pattern:

[HttpPost]
public async Task<JsonResult> QuestionPhotoPost(IFormFile FileUpload, string QuestionText, Guid? QuestionId)
{
    string TempFileName = string.Empty;
    var directiveToUpload = Path.Combine(_environment.WebRootPath, "images\\UploadFile");
    var http = HttpRequestExtensions.GetUri(Request);
    QuestionViewModel model = new QuestionViewModel();
    try
    {

        if (FileUpload != null)
        {
            TempFileName = FileUpload.FileName;
            await CheckFileFromFrontEndAsync();
        }

    }
    catch (Exception exception)
    {

    }

    async Task CheckFileFromFrontEndsync()
    {
        if (FileUpload != null)
        {
            if (!System.IO.Directory.Exists(directiveToUpload))
            {
                System.IO.Directory.CreateDirectory(directiveToUpload);
            }
            if (System.IO.File.Exists(string.Format("{0}\\{1}\\{2}", _environment.WebRootPath, "images\\UploadFile", FileUpload.FileName)))
            {
                TempFileName = Guid.NewGuid().ToString() + FileUpload.FileName;
            }
            model.PictureUrl = string.Format("{0}://{1}/{2}/{3}/{4}", http.Scheme, http.Authority, "images", "UploadFile", TempFileName);
            await SaveFileToServerAsync(TempFileName);
        }

    }

    async Task SaveFileToServerAsync(string FileName)
    {
        if (FileUpload.Length > 0)
        {
            using (var stream = new FileStream(Path.Combine(directiveToUpload, FileName), FileMode.Create))
            {
                await FileUpload.CopyToAsync(stream);
            }
        }
    }

    return Json(genericResponseObject);
}

为使代码更具可读性，我将这些内联函数移至外部.

To make the code more readable, I'd move those inline functions to outside, though.