You may come across all kinds of file extensions and still the codec can be H.264
*.avi - People also use .avi for H.264 videos too!
*.mp4 - QuickTime use this format. Better than AVI as you can store AAC audio as well.
*.m4v - The standard file format for videos for iOS devices developed by Apple.
*.mkv - (Matroska container) - can support many video and audio formats.
*.h264 - Not commonly used, but you may download some video files with this extension
name from Internet.
*MPEG-4 - One of the latest (audio and video) compression method standardized by MPEG
group, designed especially for low-bandwidth video/audio encoding purposes.