One trick on my MAC that I use quite frequently is to hit the spacebar to preview a file.  I have lots of screenshots (command+shift+4) on my desktop and they all say screenshot so it is challenging to know which one I want.  I have PowerPoints, text documents, PDF’s, videos, etc… and I tend to want to know what it is before I open it.  Single click on the file and hit the spacebar (MAC only!!) and a preview will show up. If it is a PowerPoint or Keynote you can actually cycle through the slides or in the case of a video see the video. What is great about the spacebar preview is that you do not have to wait for the program to launch for you to view the file so it is much quicker than just opening it to see what the file is.

Hint: this also works when you are browsing to attach a file in an email so you can make sure you are attaching the correct file.