Iin Opera GX 111.0.5168.54 64-bit on Windows 11 23H2, I went to https://www.tiktok.com/@tmz/video/7392341110326054190?is_from_webapp=1&sender_device=pc, right-clicked on the video and chose "Download video". The file downloaded as "Download.mp4".
In Windows Explorer (File Explorer), with "file extensions" enabled under view -> Show, is the filename really Download.html or Download.mp4.html?
Or, do you just have mp4 files associated with Opera where they have an Opera icon and double-clicking on them opens and plays them in Opera?