[jQuery] How to add method to plugin, so it's possible to call it from callbacks?

[szimek]

Hi,

I'm using MultiFile plugin and it allows you to specify callback
functions for events like i.e. afterFileAppend:

$(document).ready(function() {
  var addDescription = function(fileInput, descriptionId) {do
something};

  $('.multi_image').MultiFile({
    afterFileAppend: function(fileInput) {
      addDescription(fileInput, 'image_description');
    }
  });
});

How can I add addDescription method to MultiFile plugin, so I can use
it in all places where I use this plugin?