sequence.go
func (s *FileSequence) SetExt(ext string) {
	if !strings.HasPrefix(ext, ".") {
		ext = "." + ext
	}
	s.ext = ext
}
func (s *FileSequence) SetExt(ext string) {
	if !strings.HasPrefix(ext, ".") {
		ext = "." + ext
	}
	s.ext = ext
}
// Set a new ext for the sequence