Use VLC Media Player (it plays almost any .mp4).
📎 VID (Video) - 20181029 (Year/Month/Day) - WA (WhatsApp) - 0107 (Unique ID). VID-20181029-WA0107.mp4
Open the Photos app → Albums → Shared from Cast / WhatsApp. Use VLC Media Player (it plays almost any
Check Google Photos or iCloud for that date; WhatsApp often auto-backs up to the cloud. 💡 Pro Tip VID-20181029-WA0107.mp4
If you don't want to open the file but need to know what it is: