If you have the file and want to know what it is without opening it:
💡 Knowing the source app would help me track down the exact video.
Filenames with underscores and long digit strings are common for media shared through messaging app servers. 🛠️ How to Identify the Content
The filename appears to be a specific identifier typically used by Reddit's video hosting service (v.redd.it) or similar social media content delivery networks .
If you have the link it came from, search for the folder ID (the 28821 part) on Google to find the original post.