B_045.mp4

Messaging platforms often rename files during the "save to gallery" process if the original metadata is stripped.

It assigns a generic letter and number (e.g., b for "block" or "batch") to the recovered video. b_045.mp4

A video under 1MB is likely a low-quality meme or a "crash video" (designed to freeze your media player). Messaging platforms often rename files during the "save