File Upload Validation

Hi Ranadeep,

The extension validation is an add-on for the jQuery validation library and it's not included in Meta Box.

In order to use it, please download the extension.js file from Github:

https://github.com/jquery-validation/jquery-validation/blob/master/src/additional/extension.js

And enqueue it like this:

add_action( 'rwmb_enqueue_scripts', function() {
    wp_enqueue_script( 'jquery-validation-extension', 'url to the js file', '', '', true );
}

Then you can use the extension rule.