Here’s how to achieve what you want:
1. Disable/Uninstall the SEO Strict URLs plugin. YAMS has its own in-built in SEO strict functionality which conflicts with that plugin.
2. Set Modules>YAMS>Other Params>URL Formatting>Use Mime-type dependent suffixes? = Yes.
3. Clear your cache.
That should be it. The default MIME => suffix mapping is as follows:
$this->itsMimeSuffixMap = array(
'application/xhtml+xml' => '.xhtml'
, 'application/rss+xml' => '.rss'
, 'application/xml' => '.xml'
, 'text/xml' => '.xml'
, 'text/css' => '.css'
, 'text/html' => '.html'
, 'text/plain' => '.txt'
If you want to change it, you will have to edit the assets/modules/yams/yams.config.inc.php file directly.