in struct.go [300:311]
func (h *FileHeader) SetMode(mode fs.FileMode) {
h.CreatorVersion = h.CreatorVersion&0xff | creatorUnix<<8
h.ExternalAttrs = fileModeToUnixMode(mode) << 16
// set MSDOS attributes too, as the original zip does.
if mode&fs.ModeDir != 0 {
h.ExternalAttrs |= msdosDir
}
if mode&0200 == 0 {
h.ExternalAttrs |= msdosReadOnly
}
}