if (Request.Files.Count > 0)
{
  //这里只测试上传第一张图片file[0]
  HttpPostedFile file0 = Request.Files[0];
  string ext = file0.FileName.Substring(file0.FileName.LastIndexOf('.') + 1);//文件扩展名string[] fileTypeStr = { "jpg", "gif", "bmp", "png" };
  if (fileTypeStr.Contains(ext))
  {
    file0.SaveAs(Server.MapPath("~/" + file0.FileName));//保存文件  }
  else
  {
    Response.Write("图片格式不正确" + ext);
  }
}

if (Request.Files.Count > 0)
{
//这里只测试上传第一张图片file[0]
  HttpPostedFile file0 = Request.Files[0];
  string contentType = file0.ContentType;//文件类型string[] fileTypeStr = { "image/gif","image/x-png","image/pjpeg","image/jpeg","image/bmp"};
  if (fileTypeStr.Contains(contentType))
  {
    file0.SaveAs(Server.MapPath("~/" + file0.FileName));
  }
  else
  {
    Response.Write("图片格式不正确" + contentType);
  }
}

if (Request.Files.Count > 0)
{
//这里只测试上传第一张图片file[0]
  HttpPostedFile file0 = Request.Files[0];
  //转换成byte,读取图片MIME类型  Stream stream;
  //int contentLength = file0.ContentLength; //文件长度byte[] fileByte = newbyte[2];//contentLength，这里我们只读取文件长度的前两位用于判断就好了，这样速度比较快，剩下的也用不到。
  stream = file0.InputStream;
  stream.Read(fileByte, 0, 2);//contentLength，还是取前两位  stream.Close();
  string fileFlag = "";
  if (fileByte != null && fileByte.Length > 0)//图片数据是否为空  {
    fileFlag = fileByte[0].ToString() + fileByte[1].ToString();         
  }
  string[] fileTypeStr = { "255216", "7173", "6677", "13780" };//对应的图片格式jpg,gif,bmp,pngif (fileTypeStr.Contains(fileFlag))
  {
    file0.SaveAs(Server.MapPath("~/" + file0.FileName));
  }
  else
  {
    Response.Write("图片格式不正确:" + fileFlag);
  }
}