Filr add custom columns with example.

functions.php

<?php

// First we add the titles for the columns.
add_filter( 'filr_header_columns', function( $columns ) {

    // You can add as many columns as you like here.
	$columns['purchase-order-number'] = array(
		'title' => esc_html__( 'Purchase Order Number', 'textdomain' ),
		'hide'  => 'off',
	);

	$columns['another-column'] = array(
		'title' => esc_html__( 'Just another column.', 'textdomain' ),
		'hide'  => 'off',
	);

	return $columns;
});

// Now we assign content to our new columns.
add_action( 'filr_custom_column', function( $td, $file_id ) {
	?>
	<?php if ( 'purchase-order-number' === $td ) : ?>
        <td class="purchase-order-number">
			<?php echo get_post_meta($file_id, 'purchase_order_number', true ); ?>
        </td>
	<?php endif; ?>
	<?php if ( 'another-column' === $td ) : ?>
        <td class="another-column">
			<?php echo get_post_meta($file_id, 'another_column', true ); ?>
        </td>
	<?php endif; ?>
	<?php
}, 10, 2 );