```ruby
require 'find'

module HamlLint
  # A miscellaneous set of utility functions.
  module Utils
    class << self
      def extract_files_from(list)
        files = []

        list.each do |file|
          begin
            Find.find(file) do |f|
              files << f if haml_file?(f)
            end
          rescue Errno::ENOENT
            # One of the paths specified does not exist; raise a more
            # descriptive exception so we know which one
            raise HamlLint::Exceptions::InvalidFilePath,
                  "File path '#{file}' does not exist"
          end
        end

        files.uniq
      end

    private

      def haml_file?(file)
        return false unless FileTest.file?(file)

        File.extname(file) == '.haml'
      end
    end
  end
end
```

Use module_function instead of class << self
user
```ruby
require 'find'

module HamlLint
  # A miscellaneous set of utility functions.
  module Utils
    module_function

    def extract_files_from(list)
      files = []

      list.each do |file|
        begin
          Find.find(file) do |f|
            files << f if haml_file?(f)
          end
        rescue Errno::ENOENT
          # One of the paths specified does not exist; raise a more
          # descriptive exception so we know which one
          raise HamlLint::Exceptions::InvalidFilePath,
                "File path '#{file}' does not exist"
        end
      end

      files.uniq
    end

  private

    def haml_file?(file)
      return false unless FileTest.file?(file)

      File.extname(file) == '.haml'
    end
  end
end
```
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