For example, did you see this mentioned in a specific forum, find it on a hard drive, or receive it as a link? Knowing the source would help me track down exactly what this specific file contains.
Look at the "Date Created" or "Origin" in the file properties to see where it came from. 39518.mp4
Apps often rename files when saving them to your local device storage to prevent naming conflicts. For example, did you see this mentioned in
Large repositories or "leak" sites often use numerical IDs to catalog content anonymously. How to Identify the File Apps often rename files when saving them to
Many NVR (Network Video Recorder) systems use numerical strings to timestamp or index clips.
You can use a tool like VirusTotal to upload the file (or its MD5 hash). This will tell you if the file is known for containing malware or if it has been flagged in other databases.