Retrieving a sensible file extension for a given MIME string in Rails

Here's a little snippet of code to retrieve a sensible file extension based on a MIME string, using the built-in MIME dictionary in Rails 

  module Mime
      class Type
          class << self
              # Lookup, guesstimate if fail, the file extension
              # for a given mime string. For example:
              #
              # >> Mime::Type.file_extension_of 'text/rss+xml'
              # => "xml"
              def file_extension_of(mime_string)
                  set = Mime::LOOKUP[mime_string]
                  sym = set.instance_variable_get("@symbol") if set
                  return sym.to_s if sym
                  return $1 if mime_string =~ /(\w+)$/
              end
          end
      end
  end 

As mentioned in the comments in the code, you can now give it Mime::Type.file_extension_of 'text/rss+xml' and it will return you "xml". Any extension missing, just apply the standard Rails MIME registration steps

  Mime::Type.register "text/vnd.sun.j2me.app-descriptor", :jad
  Mime::Type.file_extension_of "text/vnd.sun.j2me.app-descriptor"  # => "jad" 

The bonus with using the standard Mime::Type.register is that both your MIME lookup and respond_to gets smarter together.