It is possible the string is part of a DOI (Digital Object Identifier) or a specific database ID. For example:
: If "21676" is a PMID, it would refer to a very old medical study from the 1940s-50s. 21676mp4
The number 21676 appears in various contexts that may have associated documentation: It is possible the string is part of
: Search for "H.264/AVC" or "H.265/HEVC" efficiency if the number refers to a specific bitrate or resolution setting. 2. Specific Technical Documents 21676mp4
If the term refers to the MP4 container or a specific encoding, you might be looking for papers on the standard.
: The official ISO page for MPEG-4 Part 14 provides the technical specifications for the MP4 file format.