Since the filename itself doesn't describe the video, you can:

: Search your "Saved Messages" or recent chats for "5021547738098041451" to find the original message.

The filename "1_5021547738098041451mp4" follows the specific naming convention used by for downloaded media files.

While I cannot access the specific video file stored on your device or within a private Telegram chat, here is the context for that file type: File Breakdown : Telegram messenger (Desktop or Mobile). Format : MP4 (standard video file).