A Ruby script that uses the `file` command to add or change file extensions for a list of files

fix-extension.rb

#!/usr/bin/ruby
require 'fileutils'

Entry = Struct.new(:path, :extension) do
  def self.parse line
    columns = line.chomp.split(/:\s*/)
    return nil unless columns.length == 2
    path, extensions = columns
    return nil if extensions == '???'
    extension = extensions.split('/').first
    new(path, extension)
  end

  def path_without_extension
    path.split('.').first
  end

  def move
    new_path = path_without_extension + '.' + extension
    FileUtils.mv(path, new_path) unless path == new_path
  end
end

entries = `file --extension #{ARGV.join(' ')}`.lines.map { Entry.parse _1 }.compact
entries.each(&:move)

Warning: May overwrite files whose paths are identical up to the first dot. May cause disastrous results for paths containing a dot in a directory name.