public void UploadFile2()
{
//获取文件值
string filename = Request.QueryString["value"];
//获取图片
string oldfilename = "wechat.png";
var contentType = MimeMapping.GetMimeMapping(oldfilename);
var bytes = mongoDBHelper.DownloadAsBytesByName(oldfilename);
//通过Metadata 添加附加信息
GridFSUploadOptions option = new GridFSUploadOptions();
option.ContentType = contentType;
mongoDBHelper.UploadFromBytes(bytes, filename, option);
ViewImg();
}