Apps often rename files when saving them to your local device storage to prevent naming conflicts.
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. 39518.mp4
A very small file (KB) is likely a short clip or meme; a large file (MB/GB) is likely a full video or high-definition recording. Apps often rename files when saving them to
If you have the file and are trying to figure out what it is without opening it, you can: find it on a hard drive