"IMG_3147.M4V" is a specific file name typically generated by (like iPhones or iPads) when recording video . Because this is a generic filename, there isn't a single "topic" for it; instead, it represents a common technical hurdle for users trying to manage, share, or play their personal media.
Upload the file to Google Drive , iCloud , or Dropbox and share the link instead of attaching the raw file.
Surprisingly, you can often just change the extension from .m4v to .mp4 manually, and most players will still read it.