Wordpress - allow any extension

functions.php

<?php
// Solution started from: https://wordpress.stackexchange.com/questions/231448/how-to-add-dot-in-post-slug
// This will allow the "." character inside urls
function allow_htm_extensions( $title, $raw_title = '', $context = 'display' ) {
    // IN HERE WE WRITE THE ACCEPTED EXTENSIONS
    $allowed_extensions = array('.htm','.html');

    $title = strip_tags($title);
    // Preserve escaped octets.
    $title = preg_replace('|%([a-fA-F0-9][a-fA-F0-9])|', '---$1---', $title);
    // Remove percent signs that are not part of an octet.
    $title = str_replace('%', '', $title);
    // Restore octets.
    $title = preg_replace('|---([a-fA-F0-9][a-fA-F0-9])---|', '%$1', $title);

    if (seems_utf8($title)) {
        if (function_exists('mb_strtolower')) {
            $title = mb_strtolower($title, 'UTF-8');
        }
        $title = utf8_uri_encode($title, 200);
    }

    $title = strtolower($title);

    if ( 'save' == $context ) {
        // Convert nbsp, ndash and mdash to hyphens
        $title = str_replace( array( '%c2%a0', '%e2%80%93', '%e2%80%94' ), '-', $title );
        // Convert nbsp, ndash and mdash HTML entities to hyphens
        $title = str_replace( array( '&nbsp;', '&#160;', '&ndash;', '&#8211;', '&mdash;', '&#8212;' ), '-', $title );

        // Strip these characters entirely
        $title = str_replace( array(
            // iexcl and iquest
            '%c2%a1', '%c2%bf',
            // angle quotes
            '%c2%ab', '%c2%bb', '%e2%80%b9', '%e2%80%ba',
            // curly quotes
            '%e2%80%98', '%e2%80%99', '%e2%80%9c', '%e2%80%9d',
            '%e2%80%9a', '%e2%80%9b', '%e2%80%9e', '%e2%80%9f',
            // copy, reg, deg, hellip and trade
            '%c2%a9', '%c2%ae', '%c2%b0', '%e2%80%a6', '%e2%84%a2',
            // acute accents
            '%c2%b4', '%cb%8a', '%cc%81', '%cd%81',
            // grave accent, macron, caron
            '%cc%80', '%cc%84', '%cc%8c',
        ), '', $title );

        // Convert times to x
        $title = str_replace( '%c3%97', 'x', $title );
    }

    $title = preg_replace('/&.+?;/', '', $title); // kill entities

    foreach($allowed_extensions as $extension) {
        if(strlen($title) > strlen($extension)) {
            $position = strpos($title, $extension, (strlen($extension) * (-1)));
            if ($position !== false) {
                $title = substr($title, 0, $position);
                $url_ext = $extension;
                break;
            }
        }
    }
    $title = str_replace('.', '-', $title);

    // WPSE-231448: Add the dot to the list of characters NOT to be stripped.
    $title = preg_replace('/[^%a-z0-9 _\-]/', '', $title);
    $title = preg_replace('/\s+/', '-', $title);
    $title = preg_replace('|-+|', '-', $title);
    $title = trim($title, '-');

    $title = (isset($url_ext)) ? $title.$url_ext : $title;

    return $title;
}