extensionsMu sync.Mutex // Guards stores (but not loads) on extensions.
	extensions   sync.Map   // map[string][]string; slice values are append-only.
)

// setMimeTypes is used by initMime's non-test path, and by tests.
func setMimeTypes(lowerExt, mixExt map[string]string) {
	mimeTypes.Clear()
	mimeTypesLower.Clear()
	extensions.Clear()

	for k, v := range lowerExt {
		mimeTypesLower.Store(k, v)
	}

	"testing"
)

func setMimeInit(fn func()) (cleanup func()) {
	once = sync.Once{}
	testInitMime = fn
	return func() {
		testInitMime = nil
		once = sync.Once{}
	}
}

func clearMimeTypes() {
	setMimeTypes(map[string]string{}, map[string]string{})
}

func setType(ext, typ string) {
	if !strings.HasPrefix(ext, ".") {
		panic("missing leading dot")
	}
	if err := setExtensionType(ext, typ); err != nil {