Developping a plugin that extend custom field Ajax File field.

-- HikaShop version -- : 3.3.0
-- Joomla version -- : 3.8.5

Guys, I ask you for guidance, please explain to me how can i extend and use class hikashopFieldAjaxfile in my own custom field plugin.
I tried to understand how the datepicker plugin is working, but without much success.
I want only to add some restriction to the files that can be uploaded through this custom field. Iknow that Hikashop has already implemented a restriction system for files, but i want to allow users to upload many types of files. But when a user sell an audio file, i want to allow him to upload 1 demo file that should be an mp3 file, or wav.
I can not add these restrictions to Hikashop's main settings,because the users can also upload images.

How can i use this:
class plgHikashopMyclass extends hikashopFieldAjaxfile { } ?
I want this field to be visible in frontend, cuz i'm using hikamarket.
This is necessary in every plugin?

function plgHikashopName(&$subject, $config){
		parent::__construct($subject, $config);
	}