I have a asp FileUpload control in my aspx page.

And the code behind to check file's extension and then insert them into database:

 private string Write(HttpPostedFile file, string table)
    {
        string fileNameMain = Path.GetFileName(file.FileName);

        // check for the valid file extension
        string fileExtension = Path.GetExtension(fileNameMain).ToLower();
        if (fileExtension.Equals(".pdf") || fileExtension.Equals(".doc"))
        {
            ...insert fileupload into database
        }
        else
        {
            LabelError.Text = "Extension of files are not allowed.";
            return null;
        }
    }

With the code above, it checks the extension and then show the message "Extension of files are not allowed." in a LabelError if the fileupload is not allowed.

Now, I'm required to do the "check-extension" in the other way: at the moment the client click and choose FileUpload, an alert show "Extension of files are not allowed.".

I need a way to make an alert show at the momment the FileUpload choosed in browser. Help!!!!