FileUpload時用Javascript檢查副檔名是否有效

View Code <script type="text/javascript">        var validFilesTypes = ["png", "jpg", "jpeg", "gif"];        function ValidateFile() {            var file = document.getElementById("<%=FileUpload1.ClientID%>");            var filePath = file.value;            var ext = filePath.substring(filePath.lastIndexOf(".") + 1, filePath.length).toLowerCase();            var isValidFile = false;            for (var i = 0; i < validFilesTypes.length; i++) {                if (ext == validFilesTypes[i]) {                    isValidFile = true;                    break;                }            }            if (!isValidFile) {                alert("無效的檔案。請上傳副檔名為：\n\n" + validFilesTypes.join(", "));            }            return isValidFile;        }    </script>
<asp:FileUpload ID="FileUpload1" runat="server" /> <asp:Button ID="btnUpload" runat="server" Text="Upload" OnClientClick="return ValidateFile()" />