无法将HttpFileCollectionBase转换为HttpFileCollection

在你的行动方面做一些这样的事情。不能将文件作为参数传递:

[HttpPost]
public ActionResult add(string TagsInput) {
  if (Request.Files.Count > 0) {
    // for this example; processing just the first file
         HttpPostedFileBase file = Request.Files[0];
 if (file.ContentLength == 0) {
      // throw an error here if content length is not > 0
      // you'll probably want to do something with file.ContentType and file.FileName
      byte[] fileContent = new byte[file.ContentLength];
      file.InputStream.Read(fileContent, 0, file.ContentLength);
      // fileContent now contains the byte[] of your attachment...
    }  
  }
  return View();
}