Hi mvjr,
Please use property "FileTooLargeMsg" and "MaxFilesLimitMsg".
<CuteWebUI:UploadAttachments runat="server" ID="Uploader1" MaxFilesLimit="1" FileTooLargeMsg="file too large" MaxFilesLimitMsg="support 1 file a time">
<ValidateOption MaxSizeKB="10" />
</CuteWebUI:UploadAttachments>
Regards,
Ken