File Extensions by MIME Type

mime.rb

require 'mime/types'

ARGV.each do |mime_type|
  mimes  = MIME::Types[mime_type]
  exts = []
  mimes && mimes.each do |mime|
    mime.extensions.each { |x| exts << x }
  end
  puts "#{mime_type} extensions : #{exts.join(',')}"
end