This shows a folder icon. The fa-2x is a standard FontAwesome class which makes the icon twice as big as the normal font size. The dm-tree2-icon used in the example here defines a color for the icon so that it can be purple instead of black.
To add a FA icon to a button accepting an icon use the FontAwesome name without any embellishments, like:
To be done
Support for FA icons in button(s).