This specific naming convention ( AgAD... ) is a unique file ID generated by Telegram.
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.
Since the filename is randomized, you cannot determine the content by the name alone. To see what is in the file, you can:
The .mkv extension indicates it is a Matroska Video file, which is a high-definition container format capable of holding multiple audio tracks, subtitles, and high-quality video. How to Identify the Actual Content
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.
Right-click the file and select Properties (Windows) or Get Info (Mac). Sometimes the "Details" tab will contain the original title or tags.