Fix #362, update code with upstream

modules/base/markdown.go

@@ -39,7 +39,7 @@ func isLink(link []byte) bool {
 func IsMarkdownFile(name string) bool {
 	name = strings.ToLower(name)
 	switch filepath.Ext(name) {
-	case ".md", ".markdown", ".mdown":
+	case ".md", ".markdown", ".mdown", ".mkd":
 		return true
 	}
 	return false