Add custom columns to FILR libraries

functions.php

<?php

add_filter( 'filr_header_columns', function( $columns ) {
    $columns['custom'] = array(
	    'title' => esc_html__( 'Custom', 'filr' ),
	    'hide'  => 'off',
    );

    return $columns;
});

add_action( 'filr_custom_column', function( $td, $file_id ) {
    ?>
	<?php if ( 'custom' === $td ) : ?>
        <td class="custom">
            <?php echo $file_id; ?>
        </td>
    <?php endif; ?>
    <?php
}, 10, 2 );

Make sure you add the newly registered header column in FILR -> Settings -> Sort your columns.