@@ -10,6 +10,8 @@ module Engine
 DEFAULT_IDENTIFIER = OpenStruct.new(value: "parse-error")
 
 class CSSlint
+DEFAULT_EXTENSIONS = [".css"].freeze
+
 autoload :CheckDetails, "cc/engine/csslint/check_details"
 
 def initialize(directory: , io: , engine_config: )
@@ -33,6 +35,8 @@ def run
 
 private
 
+attr_reader :engine_config
+
 # rubocop:disable Metrics/MethodLength
 def create_issue(node, path)
 check_name = node.attributes.fetch("identifier", DEFAULT_IDENTIFIER).value
@@ -72,14 +76,16 @@ def csslint_xml
 end
 
 def files_to_inspect
-include_paths = @engine_config["include_paths"] || ["./"]
-include_paths.each_with_object([]) do |path, out|
+include_paths = engine_config.fetch("include_paths", ["./"])
+extensions = engine_config.fetch("extensions", DEFAULT_EXTENSIONS)
+extensions_glob = extensions.join(",")
+include_paths.flat_map do |path|
 if path.end_with?("/")
-out.concat(Dir.glob("#{path}**/*.css"))
-elsif path.end_with?(".css")
-out << path
+Dir.glob("#{File.expand_path path}/**/*{#{extensions_glob}}")
+elsif path.end_with?(*extensions)
+path
 end
-end
+end.compact
 end
 end
 end

@@ -75,6 +75,23 @@ module Engine
 expect{ lint.run }.not_to output(/config.yml/).to_stdout
 end
 end
+
+describe "with custom extensions" do
+let(:engine_config) do
+{
+"extensions" => %w(.fancycss)
+}
+end
+
+before do
+create_source_file("master.fancycss", id_selector_content)
+end
+
+it "takes into account extensions" do
+expect{ lint.run }.to \
+output(/Don't use IDs in selectors./).to_stdout
+end
+end
 end
 
 def create_source_file(path, content)