<asp:FileUpload ID="FileUpload1" runat="server" />
<asp:Button ID="btnUpload" runat="server" Text="Upload" OnClientClick="return ValidateFile()" />

<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("无效的文件。请上传文件扩展名为：\n\n" + validFilesTypes.join(", "));
}

return isValidFile;
}
</script>