You can upload suspicious files to VirusTotal to scan them against multiple antivirus engines before opening.
If you did not intentionally download this file, do not open it. Download File WaterMark1621357830978.mp4
If the file was downloaded from a service like Google Drive or Dropbox and appears as a generic "File" without the .mp4 extension, you can often fix this by manually renaming it to add .mp4 to the end. How to play MP4 files in Windows - Microsoft Support You can upload suspicious files to VirusTotal to
On Windows or macOS, you can typically open an MP4 by double-clicking it. If it doesn't play, you may need a versatile media player like VLC Media Player or the Windows Media Player . How to play MP4 files in Windows -
If you have encountered this file on your device or a website, consider the following: