HttpFileCollection 上的枚举器返回文件的键(名称)，而不是 HttpPostedFileBase 对象.获得密钥后，使用带有密钥(文件名)的 Item ([]) 属性来获取 HttpPostedFileBase 对象.

The enumerator on the HttpFileCollection returns the keys (names) of the files, not the HttpPostedFileBase objects. Once you get the key, use the Item ([]) property with the key (filename) to get the HttpPostedFileBase object.

foreach (string fileName in Request.Files)
{
    HttpPostedFileBase file = Request.Files[fileName];

    ...
}