Force QuickTime Player to auto-play files
Just use Quick Look instead. Click on the video file to select it. Then tap your space-bar, and the video will appear in Quick Look. Watch it however you wish to. Then when you're finished, just tap your space-bar again and you're done.
Using the default will command QuickTime to start playback of the loaded file.
defaults write com.apple.QuickTimePlayerX MGPlayMovieOnOpen 1
It's not file specific, or movie specific. It will play all files, regardless of type if you run this command, and restart QuickTime.