我在我的项目中使用webapi上传文件。我正在用Postman进行测试。但是，**Request.Content.IsMimeMultipartContent（）**总是返回false。
Postman 截图：

文件上传控制器代码：

public async Task<HttpResponseMessage> UserImageUpload()
    {
        try
        {
            if (!Request.Content.IsMimeMultipartContent())
            {
                throw new HttpResponseException(HttpStatusCode.UnsupportedMediaType);
            }

            var userImageUploadPath = HttpContext.Current.Server.MapPath(CommonParameters.UserProfileImageServerPath);
            var streamProvider = new CustomMultipartFormDataStreamProvider(userImageUploadPath);
            await Request.Content.ReadAsMultipartAsync(streamProvider);

            var files = new List<string>();
            foreach (MultipartFileData file in streamProvider.FileData)
            {
                files.Add(Path.GetFileName(file.LocalFileName));
            }

            return Request.CreateResponse(HttpStatusCode.OK, files);
        }
        catch (Exception exception)
        {
            logger.ErrorFormat("An error occured in UserImageUpload() Method - Class:FileUploadController - Message:{0}", exception);
            return Request.CreateResponse(HttpStatusCode.BadRequest);
        }
    }