guess mime type from filename extension

gistfile1.rb

    # Internal: Lookup mime type for extension.
    #
    # Returns a MIME::Type
    def _mime_type
      if defined? @_mime_type
        @_mime_type
      else
        guesses = ::MIME::Types.type_for(extname.to_s)

        # Prefer text mime types over binary
        @_mime_type = guesses.detect { |type| type.ascii? } ||
          # Otherwise use the first guess
          guesses.first
      end
    end