This specific naming convention ( AgAD... ) is a unique file ID generated by Telegram.
Use a player like VLC Media Player or MPC-HC , which are designed to handle .mkv files regardless of their filename. AgADtAIAAkXhEEQ.mkv
Since the filename is randomized, you cannot determine the content by the name alone. To see what is in the file, you can: This specific naming convention ( AgAD