In my previous tutorial, I’d explained how to upload and download files from the server folder in asp.net, how to send test mail using gmail smtp settings, how to export gridview selected rows to word excel text and pdf and other more cracking tutorials on Asp.net, JavaScript, jQuery here. Now here in this tutorial, I’ll explain how to check or validate file extensions during/before file upload from client-side using javascript and from server-side using custom validator. Like say for example, if you want to allow users to upload only images, you can allow users to upload only files with .jpg, .jpeg, .png, .gif extensions.