require_once(ABSPATH . 'wp-admin/includes/image.php');

$filename = 'your file name';

$attachment = array(
    'post_mime_type' => 'your mime type',
    'post_title' => preg_replace('/.[^.]+$/', '', basename($filename)),
    'post_content' => '',
    'post_status' => 'inherit',
    'guid' => $wp_upload_dir['url'] . '/' . basename($filename)
);
$attach_id = wp_insert_attachment( $attachment, $filename,'your_post_id' );
$attach_data = wp_generate_attachment_metadata( $attach_id, $filename );
wp_update_attachment_metadata( $attach_id, $attach_data );
update_post_meta($id, '_thumbnail_id', $attach_id);