Add xsl xslt mimetype to nginx mime.types

Upstream nginx lacks any mime.types configuration for xsl and xslt. This means that a default nginx installation serves xsl and xslt files with the default mimetype of application/octet-stream, which will break most processors.

I have created a ticket for this issue in upstream nginx. https://trac.nginx.org/nginx/ticket/2552

It is true that the mimetypes can be quite easily customized by adding a

types {
  my/mimetype    ext
} 

directive within the http{} context of nginx config.

It is also equally true that taking on the burden of correctly supporting and serving every mimetype known to humankind would be a nightmare.

XSL and XSLT however are well known document types and are not unusual in Web applications.

And while it is true that an admin can add these kinds of customizations rather easily to the server, in a world of containerized deployments, that requires finicky manouvers in Dockerfiles or other build tools to awk or sed your way into having a working config. So much nicer if nginx just works out of the box.

I note also that debian ships with application/xslt+xml xsl xslt; in its /etc/mime.types

I suppose an alternative suggestion would have been to break ranks with upstream nginx and find a way to build nginx's custom mime.types file from the /etc/mime.types file while packaging nginx. That would also satisfy this request and would auto-align nginx's mimetypes with Debian's shipping mimetypes.