When mime types are registered they are placed in a hash constant EXTENSION_LOOKUP in the module Mime. For reference, the file with the relevant code is in rails/action_pack/lib/action_dispatch/http/mime_type.rb available on Github at https://github.com/rails/rails/blob/master/actionpack/lib/action_dispatch/http/mime_type.rb

You can check for existence of certain mime types with

Mime::Type.lookup_by_extension(:json)

which will return the object or nil if it's not registered. You can also iterate over all registered mime types by cycling through the constant like so

Mime::EXTENSION_LOOKUP.each { |m| puts m}

Api documentation is here: http://api.rubyonrails.org/classes/Mime/Type.html#method-c-lookup_by_extension

Just finishing up brewing up some fresh ground comments...