: Ensure it is actually an .mp4 and not a double-extension file like BOYST0250.mp4.exe , which could be a virus. To give you more specific advice, could you tell me: Where did you find this file?

: Run it through a scanner like VirusTotal before opening.

If your standard media player won't open it, the file might use a specific codec or be slightly corrupted.

: It may be a clip from a specific stock footage library or a private backup.

Are you having , or do you just want to know what is in it ? Is it part of a larger collection of similar files?

If you need to know exactly what is inside the file (resolution, frame rate, bitrate), use a tool like MediaInfo . This will tell you: : (e.g., H.264, H.265/HEVC) Audio Channels : (e.g., Stereo, AAC)