It turns out that IANA (the Internet Assigned Numbers Authority) is also responsible for registering MIME types.
You can also go to IANA to find out who/what owns various top-level domains (TLDs) and the various ccTLDs (country code TLDs). My dreams of registering canada.eh were dashed when I found out that Western Sahara owns .eh. And they don’t have a registry.
What was I doing? I was working on a PHP script that will set a file for download, setting the header “Content-Type” to the specific type of file. In my case, a comma-separated value (CSV) file. Closest thing is application/vnd.ms-excel, which is probably where I want it to end up anyways.