Download and place the files in a directory called StlHandler in your extensions/ folder.
Add the following code at the bottom of your LocalSettings.php:
wfLoadExtension( 'StlHandler' );
$wgFileExtensions[] = 'stl'; 
$wgTrustedMediaFormats[] = "application/sla";
Add the following line:

application/sla stl

to the beginning of the file mime.types located in the includes/ folder

To users running MediaWiki 1.24 or earlier:

The instructions above describe the new way of installing this extension using wfLoadExtension() If you need to install this extension on these earlier versions (MediaWiki 1.24 and earlier), instead of wfLoadExtension( 'StlHandler' );, you need to use:

require_once "$IP/extensions/StlHandler/StlHandler.php";