Use below rewrite rule to access files which are converted from html to .shtml

RewriteRule ^(.*).html /$1.shtml [R=301,L]

AddType application/x-httpd-php .php .htm .html
AddHandler x-httpd-php .php .htm .html
<FilesMatch “\.(htm|html|php)$”>
SetHandler application/x-httpd-php5
</FilesMatch>
RedirectMatch 301 (.*)\.html$ http://www.domain.tld$1.shtml