```ruby
# Frozen-string-literal: true
# Copyright: 2012 - 2018 - MIT License
# Encoding: utf-8

require "jekyll/assets"

module Jekyll
  module Assets
    module Plugins
      class ImageOptim < Proxy
        arg_keys :optim
        content_types "image/webp"
        content_types "image/jpeg"
        content_types "image/svg+xml"
        content_types "image/tiff"
        content_types "image/bmp"
        content_types "image/gif"
        content_types "image/png"

        class UnknownPresetError < RuntimeError
          def initialize(name)
            "Unknown image_optim preset `#{name}'"
          end
        end

        class MultiplePredefinedPresetsSpecified < RuntimeError
          def initialize
            super "Specifying multiple pre-defined presets " \
              "at the same time is not supported"
          end
        end

        def process
          optimc = @env.asset_config[:plugins][:img][:optim]
          preset = @args[:optim].keys
          if preset.count > 1
            raise MultiplePredefinedPresetsSpecified
          end

          preset = preset.first
          # rubocop:disable Metrics/LineLength
          raise UnknownPreset, preset if preset != :default && !optimc.key?(preset)
          oc = optimc[preset] unless preset == :default
          optim = ::ImageOptim.new(oc || {})
          optim.optimize_image!(@file)
          @file
        end
      end
    end
  end
end

# rubocop:enable Metrics/LineLength
Jekyll::Assets::Hook.register :config, :before_merge do |c|
  c.deep_merge!({
    plugins: {
      img: {
        optim: {
          # Your config here.
        },
      },
    },
  })
end
```

Use a regexp for the content type.
user
```ruby
# Frozen-string-literal: true
# Copyright: 2012 - 2018 - MIT License
# Encoding: utf-8

require "jekyll/assets"

module Jekyll
  module Assets
    module Plugins
      class ImageOptim < Proxy
        content_types %r!^image/(?\!x-icon$)[a-zA-Z0-9\-_\+]+$!
        arg_keys :optim

        class UnknownPresetError < RuntimeError
          def initialize(name)
            "Unknown image_optim preset `#{name}'"
          end
        end

        class MultiplePredefinedPresetsSpecified < RuntimeError
          def initialize
            super "Specifying multiple pre-defined presets " \
              "at the same time is not supported"
          end
        end

        def process
          optimc = @env.asset_config[:plugins][:img][:optim]
          preset = @args[:optim].keys
          if preset.count > 1
            raise MultiplePredefinedPresetsSpecified
          end

          preset = preset.first
          # rubocop:disable Metrics/LineLength
          raise UnknownPreset, preset if preset != :default && !optimc.key?(preset)
          oc = optimc[preset] unless preset == :default
          optim = ::ImageOptim.new(oc || {})
          optim.optimize_image!(@file)
          @file
        end
      end
    end
  end
end

# rubocop:enable Metrics/LineLength
Jekyll::Assets::Hook.register :config, :before_merge do |c|
  c.deep_merge!({
    plugins: {
      img: {
        optim: {
          # Your config here.
        },
      },
    },
  })
end
```
assistant