: 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.
: Files starting with an underscore or period are often treated as hidden or system-specific files. If you found this in a system folder, it is likely a pointer or index file for a video gallery. How to handle this file: _m_smp4
: If this appeared after using a specific downloader or editing tool, check that software's "Export" or "Save" settings to see if the process was interrupted. : Use a tool like MediaInfo or VLC
Based on common file naming patterns, here are the most likely explanations for what you are seeing: How to handle this file: : If this
Could you tell me or what software you were using when it appeared? This will help me give you a much more specific answer.