Hello, How can i add extensional attribute for system files?

define extensional

sorry extended.

You can extend the File model the same as you would any other model. Unfortunately though, you'll need to write your own form widget to add extra fields to the file details popup...

https://github.com/octobercms/october/blob/master/modules/backend/formwidgets/fileupload/partials/_config_form.htm

For some reason this form isn't rendered through the normal form widget, and therefor is not eligible to be extended in the normal ways.