Make it easier to add custom MimeMappings

[philwebb]

#38900 reminded me that it's not easy to add additional mime types to the web server. It would be nice if this was possible using properties, or at the very least documented.

[bondress]

I've been doing some research on this and I've found some interesting stuff. Can this be assigned to me, please?

[scottfrederick]

Closing in favor of #39430