fix panic is format is nil

file.go

@@ -37,9 +37,10 @@ func (s *Settings[c]) HasFile(file string) bool {
 func (s *Settings[C]) AddFile(file string, format ...Format) *Settings[C] {
 
 	var f Format
-	f = RecogniseFormat
 
-	if len(format) <= 1 {
+	if format == nil || len(format) == 0 {
+		f = RecogniseFormat
+	} else if format != nil && len(format) == 1 {
 		f = format[0]
 	} else {
 		panic("too many arguments")