将post请求发送到上传文件时，HttpContext.Current.Request.Files为null

Question

尝试使用以下代码将文件发布到api时：

public async Task UploadAcceptancePageSignaturesAsync(int scheduledWorkId, string acceptanceGuid, string operativeName, bool supervisorSignature, byte[] imageFile)
{
    try
    {
        var content = new MultipartFormDataContent();
        content.Add(new StreamContent(new MemoryStream(imageFile)), "\"file\"", "\"image.png\"");
        var httpClient = new HttpClient();

        var uri = _serverUrl + "api/Acceptance/UploadSignatures/" + scheduledWorkId + "/" + acceptanceGuid + "/" + operativeName + "/" + supervisorSignature;
        var uploadServiceBaseAddress = uri;

        var httpResponseMessage = await httpClient.PostAsync(uploadServiceBaseAddress, content);

        var result = await httpResponseMessage.Content.ReadAsStringAsync();

        var x= result;

    }
    catch (Exception e)
    {
        Console.WriteLine(e);
    }
}

在服务器api端：

var imageFile = HttpContext.Current.Request.Files.Count > 0 ? HttpContext.Current.Request.Files[0] : null;

if (!ModelState.IsValid)
    return BadRequest(ModelState);

if (imageFile == null || imageFile.ContentLength <= 0)
    return BadRequest("Empty signature image");

imageFile大部分时间都是null，为什么会这样？

谢谢！