You can try the following code:
<CuteWebUI:Uploader runat="server" ID="Uploader1" InsertText="Upload"
OnFileUploaded="Uploader_FileUploaded" FileTooLargeMsg="You can customize this msg.{0} cannot be uploaded!
File size ({1}) is too large. The maximum file size allowed is set to: {2}."
MaxFilesLimitMsg="Please customize msg {0}.">
<ValidateOption AllowedFileExtensions="jpeg,jpg,gif,png" MaxSizeKB="100" />
</CuteWebUI:Uploader>
Thank you for asking