: Use a tool like MediaInfo or VLC Media Player to "Open" the file; these can often read the internal header to tell you the true codec, even if the file extension is non-standard.
Could you tell me or what software you were using when it appeared? This will help me give you a much more specific answer. _m_smp4
: Many mobile apps (like WhatsApp, Instagram, or TikTok) and video editors use underscores and specific codes to denote temporary cache files or fragmented MP4 parts during an upload or download process. The m often stands for "mobile" or "media," and smp4 is likely a shorthand for "segmented MP4." : Use a tool like MediaInfo or VLC