codec.go
func (reg *Registry) Lookup(mimetype string) (codec Codec, ok bool) {
	reg.mutex.RLock()
	codec, ok = reg.codecs[mimetype]
	reg.mutex.RUnlock()
	return
}
func (reg *Registry) Lookup(mimetype string) (codec Codec, ok bool) {
	reg.mutex.RLock()
	codec, ok = reg.codecs[mimetype]
	reg.mutex.RUnlock()
	return
}
// Lookup returns the codec associated with mimetype, ok is set to true or false
// based on whether a codec was found.