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)
	}