感谢大家，不吝赐教，还给个demo。

开发用花生壳，返代，报错400.

无奈找了个云服务，测试了下，完全ok。

这里贴一下netcore demo吧

[HttpPost]
       [Route("postfile")]
       public string UploadAsync()
       {
           try
           {
               var files = HttpContext.Request.Form.Files;
               if (files.Count < 0)
                   return "失败";
               foreach (var formFile in files)
               {
                   var filePath = @"C:\1\" + formFile.FileName.Substring(formFile.FileName.LastIndexOf("\\") + 1);
 
                   if (formFile.Length > 0)
                   {
                       using (var stream = new FileStream(filePath, FileMode.Create))
                       {
                           formFile.CopyToAsync(stream);
                       }
                   }
               }
               return "成功";
           }
           catch (Exception ex)
           {
               return "失败2";
           }
 
 
       }