We don't want to manage a list of constants on `Mime::`.  Managing
constants is strange because it will break method caches, not to mention
looking up by a constant could cause troubles.  For example suppose
there is a top level constant `HTML`, but nobody registers the HTML mime
type and someone accesses `Mime::HTML`.  Instead of getting an error
about how the mime type doesn't exist, instead you'll get the top level
constant.

So, instead of directly accessing the constants, change this:

  Mime::HTML

To this:

  Mime::Type[:HTML]