Based on the alphanumeric string (e.g., "AgADC..."), this looks like a often used by messaging apps (like Telegram ) or cloud storage platforms to name uploaded files. What you can do next:

If you have access to the file, information like the file size , duration , or creation date could help narrow down what it might be.

To help me provide a useful report, could you clarify the context of this file? For example:

If this file is from an unknown source, do not open it . MKV files can occasionally be used to hide scripts or malware if the media player has vulnerabilities.