@D V Sandeep 

This is the working solution which worked for me fine

onChange catalog client script on Attachment type variable

function onChange(control, oldValue, newValue, isLoading) {
    if (isLoading || newValue == '') {
        return;
    }

    g_form.hideFieldMsg('variableName');
    var ga = new GlideAjax('ValidateAttachmentFileName');
    ga.addParam('sysparm_name', 'checkFileName');
    ga.addParam('sysparam_attSysId', newValue);
    ga.getXMLAnswer(function(answer) {
        if (answer.toString() == 'invalid') {
            g_form.showFieldMsg('variableName', 'Invalid file name', 'error');
        }
    });
}

Script Include: It should be client callable

var ValidateAttachmentFileName = Class.create();
ValidateAttachmentFileName.prototype = Object.extendsObject(AbstractAjaxProcessor, {

    checkFileName: function() {

        var badChars = /[%$@()&<>'"]/;
        var sysId = this.getParameter('sysparam_attSysId');
        var gr = new GlideRecord("sys_attachment");
        gr.addQuery("sys_id", sysId);
        gr.query();
        if (gr.next()) {
            if (badChars.test(gr.file_name.toString()))
                return 'invalid';
            else
                return 'valid';
        }
    },

    type: 'ValidateAttachmentFileName'
});

Output: Working fine

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