I don't know if you all know, but when WeChat transmits any file, even if it's the same file, it makes a full copy each time. When the seven-day file shows as expired, the file is actually still on the phone, just not visible to you. If your friend sends it again at this time, it will still make a full copy and store it on the phone. This is why WeChat takes up so much space; isn't this a technical flaw?