This specific naming convention ( AgAD... ) is a unique file ID generated by Telegram.
If you received this file from an unknown source or a suspicious bot, be cautious about opening it, as randomized filenames are sometimes used to mask the true nature of content.
Use a player like VLC Media Player or MPC-HC , which are designed to handle .mkv files regardless of their filename.
If you found this in your device's "Telegram Documents" or "Telegram Video" folder, it is likely a video that was downloaded but not explicitly "saved to gallery/downloads" with its original name.