Some time you need to capture screenshot in Mac and want to save in specific file format. On Mac OS X the default file format for screen shot is PNG but if you want that your Mac must save screen shots in any other file format for example as JPG, GIF or PDF then just apply the following method to do so.
How to change default Mac screenshot file format
Click on Finder
Navigate to Applications > Utilities and launch Terminal
Type the following command in Terminal and hit Enter key
defaults write com.apple.screencapture type JPG
you can specify different file formats like GIF, PDF, etc as we specify JPG above in screen shot.
Now, restart your system and take a screen shot and check its extension, that’s set to JPG. In this way you can revert the screen shot file format just by typing a command in Terminal