Hi Ardalan,

1. set the mime type like below

  <ValidateOption AllowedFileMimeTypes="image/gif,image/jpeg" />

2. The mime type error message is "The file is corrupt or has an incorrect extension. ", you can catch this message link below, then shows your custom error.

<script type="text/javascript">
    function CuteWebUI_AjaxUploader_OnError(msg) {
        if (msg.indexOf("The file is corrupt or has an incorrect extension") != -1) {
            //show the message you want
            alert("The file is locked");
            return false;
        }
    }
</script>
 

Regards,

Ken