Since release Files Gallery 0.8.0, you can add your own custom icons from Material Design Icons. To do this, you will need to get the <path d="...">
attribute data from the icon you wish to use via browser inspector.
Then you would add the path data to the custom context menu icon
property:
icon: 'M13,9H11V7H13M13,17H11V11H13M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10,10 0 0,0 12,2Z`,
A bit tedious, but this is how icons are used in Files Gallery, and therefore how we need to add custom icons.