JQuery validation-input text field to be required if checkbox checked

You can add your own custom validation methods to handle things like this:

$.validator.addMethod("requiredIfChecked", function (val, ele, arg) {
    if ($("#startClientFromWeb").is(":checked") && ($.trim(val) == '')) { return false; }
    return true;
}, "This field is required if startClientFromWeb is checked...");


$("#franchiseForm").validate({  
        rules: {  
            mimeType: { requiredIfChecked: true }  
        }  
 });

Validation will not triger if input is disabled. You could use that fact - let textbox be required, but initially disabled, and enable it only when checkbox is checked.

$(function () {
   $('#startClientFromWeb').change(function () {
        if ($(this).is(':checked')) {
            $('#mimeType').removeAttr('disabled');                
        }
        else {
            $('#mimeType').attr('disabled', 'disabled');
        }
    });
});