An explicit plugin registry approach (instead of implicit descendant tracking)

handler_pattern.rb

class Handler

  def self.handles(ext = nil, &block)
    if ext
      check = -> (file) { File.extname(file.path) == ".#{ext}" }
    else
      check = block
    end
    Handler.registry[check] = self
  end
  
  def self.registry
    @registry ||= {}
  end

  def self.handlers
    registry.values
  end

  def self.handler_for(file)
    registry.each do |check, handler|
      return handler if check.(file)
    end
    nil
  end

  def self.process(file)
    handler = handler_for(file)
    if handler
      handler.new(file).process
    else
      nil
    end
  end

  def initialize(file)
    @file = file
  end

  # Abstract; override
  def process
  end

end

class PDFHandler < Handler
  handles :pdf
end

class SmallMovieHandler < Handler
  handles do |file|
    File.extname(file) == '.mov' && File.size(file) < 1024
  end
end

result = Handler.process(some_file)

plugin_examples.rb

class Plugin

  def self.register(name)
    Plugin.registry[name] = self
  end
  
  def self.registry
    @registry ||= {}
  end

  def self.plugins
    registry.values
  end

  def self.lookup(name)
    registry[name]
  end

end

# Not registered (abstract)
class Formatter < Plugin
end

class PDFFormatter < Formatter
  register :pdf_formatter
end

class PlainPlugin < Plugin
  register :plain
end