For Apache, MIME types are added by adding entries to a .htaccess file.
Apache will has a root .htaccess file. On shared hosts, this file is often not accessible. For such cases, Apache can merge multiple .htaccess files. Simple create a file named .htaccess in your folder which Apache uses to serve web content in the folder or any folder above where you will put your .iwml and .nut files.
Add the following entries:
AddType text/iwml iwml AddType text/acorn nut
Depending on your configuration, you may need to restart Apache for the changes to take effect. This is not necessary in most cases unless the root .htacess file is modified.