asp.net C# FileUpload 控件如何判断文件名的后缀

第一、
<asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" ControlToValidate="FileUpload1"
ErrorMessage="必须是 jpg或者gif文件" ValidationExpression="^(([a-zA-Z]:)|(\\{2}\w+)\$?)(\\(\w[\w].*))+(.jpg|.Jpg|.gif|.Gif)$"></asp:RegularExpressionValidator>

第二、
String fileExtension =
System.IO.Path.GetExtension(FileUpload1.FileName).ToLower();
String[] allowedExtensions =
{ ".gif", ".png", ".jpeg", ".jpg" };
for (int i = 0; i < allowedExtensions.Length; i++)
{
if (fileExtension == allowedExtensions[i])
{
fileOK = true;
}
}