...
    if ($selected_dir) {
        $selected_dir = ltrim($selected_dir, '/');
        $abs_path = dav_resolve_path($selected_dir);
        if (!is_dir($abs_path)) {
            echo "<p style='color:red;'>Directory not found: <code>$abs_path</code></p>";
        } else {
            dav_render_folder_tree($abs_path);
        }
    }
    echo '</div>';
}

function dav_render_folder_tree($base_dir) {
    $recursive = get_option('dav_recursive', 0);
    $link_files = get_option('dav_link_files', 0);
    $attributes = (array)get_option('dav_show_attributes', ['name', 'size', 'modified']);
    $filter_types = $_POST['file_types'] ?? [];

    echo '<form method="post">';
    echo '<input type="hidden" name="selected_base_dir" value="' . esc_attr($_POST['selected_base_dir']) . '">';
    echo '<p><strong>Filter by File Type:</strong><br>';
    $all_exts = dav_detect_extensions($base_dir, $recursive);
    foreach ($all_exts as $ext) {
        $checked = in_array($ext, $filter_types) ? 'checked' : '';
        echo "<label><input type='checkbox' name='file_types[]' value='$ext' $checked> .$ext</label> ";
    }
    echo '</p><input type="submit" class="button" value="Filter"></form>';

    $files = dav_list_files($base_dir, $recursive, $filter_types);
    if (!$files) {
        echo '<p>No matching files found.</p>';
        return;
    }

    echo '<div class="dav-flex-table">';
    echo '<div class="dav-flex-row header">';
    foreach ($attributes as $attr) {
        echo '<div class="col ' . esc_attr($attr) . '">' . ucfirst($attr) . '</div>';
    }
    echo '</div>';

    foreach ($files as $file) {
        echo '<div class="dav-flex-row">';
        foreach ($attributes as $attr) {
            switch ($attr) {
                case 'name':
                    $name = basename($file);
                    $url = dav_relative_url($file);
                    if ($link_files) {
                        echo '<div class="col name"><a href="' . esc_url($url) . '" target="_blank">' . esc_html($name) . '</a></div>';
                    } else {
                        echo '<div class="col name">' . esc_html($name) . '</div>';
                    }
                    break;
                case 'size':
                    echo '<div class="col size">' . size_format(filesize($file)) . '</div>';
                    break;
                case 'modified':
                    echo '<div class="col modified">' . date("Y-m-d H:i:s", filemtime($file)) . '</div>';
                    break;
                case 'type':
                    echo '<div class="col type">' . strtoupper(pathinfo($file, PATHINFO_EXTENSION)) . '</div>';
                    break;
            }
        }
        echo '</div>';
    }
    echo '</div>';
}

function dav_detect_extensions($dir, $recursive = false) {
    $exts = [];
    $files = dav_list_files($dir, $recursive);
    foreach ($files as $file) {
        $ext = strtolower(pathinfo($file, PATHINFO_EXTENSION));
        if ($ext && !in_array($ext, $exts)) {
            $exts[] = $ext;
        }
    }
    return $exts;
}
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /usr/home/stepitup/public_html/marybethmantiply.com/wp-content/plugins/directory-admin-viewer/directory-admin-viewer.php:1) in <b>/usr/home/stepitup/public_html/marybethmantiply.com/wp-includes/sitemaps/class-wp-sitemaps-renderer.php</b> on line <b>126</b><br />
<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href="https://www.marybethmantiply.com/wp-sitemap-index.xsl" ?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"><sitemap><loc>https://www.marybethmantiply.com/wp-sitemap-posts-post-1.xml</loc></sitemap><sitemap><loc>https://www.marybethmantiply.com/wp-sitemap-posts-page-1.xml</loc></sitemap><sitemap><loc>https://www.marybethmantiply.com/wp-sitemap-taxonomies-category-1.xml</loc></sitemap><sitemap><loc>https://www.marybethmantiply.com/wp-sitemap-users-1.xml</loc></sitemap></sitemapindex>