diff --git a/extensions.go b/extensions.go
index 17757ed..4996ea3 100644
--- a/extensions.go
+++ b/extensions.go
@@ -1,6 +1,9 @@
 package main
 
-import "path/filepath"
+import (
+	"path/filepath"
+	"strings"
+)
 
 var extensions = map[string]bool{
 	".htm":  true,
@@ -12,3 +15,7 @@ func isMarkupFile(f string) bool {
 	_, ok := extensions[filepath.Ext(f)]
 	return ok
 }
+
+func isHTMLFile(f string) bool {
+	return strings.HasSuffix(f, ".html") || strings.HasSuffix(f, ".htm")
+}
diff --git a/extensions_test.go b/extensions_test.go
index be29e91..a3314d5 100644
--- a/extensions_test.go
+++ b/extensions_test.go
@@ -15,3 +15,13 @@ func TestIsMarkupFile(t *testing.T) {
 		assert.False(t, isMarkupFile(f))
 	}
 }
+
+func TestIsHTMLFile(t *testing.T) {
+	for _, f := range []string{"foo.html", "foo.htm", "foo/bar.html"} {
+		assert.True(t, isHTMLFile(f))
+	}
+
+	for _, f := range []string{"foo", "foo.md", "bar/foo"} {
+		assert.False(t, isHTMLFile(f))
+	}
+}
diff --git a/file_checker.go b/file_checker.go
index 93792cb..3b08546 100644
--- a/file_checker.go
+++ b/file_checker.go
@@ -4,7 +4,6 @@ import (
 	"bytes"
 	"io/ioutil"
 	"net/url"
-	"strings"
 	"sync"
 	"time"
 
@@ -122,7 +121,3 @@ func isURL(s string) bool {
 	u, err := url.Parse(s)
 	return err == nil && (u.Scheme == "" || u.Scheme == "http" || u.Scheme == "https")
 }
-
-func isHTMLFile(f string) bool {
-	return strings.HasSuffix(f, ".html") || strings.HasSuffix(f, ".htm")
-}