Using attachment_fields_to_edit filter inside plugin class

If this is really your code:

<?php
add_filter('attachment_fields_to_edit', array($this, 'attachment_fields', 15, 2));

The reason it doesn’t work is because of a misplaced parenthesis. Try this:

<?php
add_filter('attachment_fields_to_edit', array($this, 'attachment_fields'), 15, 2);