Change Default Screenshot File Type

open Terminal and fill command

JPG

defaults write com.apple.screencapture type jpg

PDF

defaults write com.apple.screencapture type pdf

TIFF

defaults write com.apple.screencapture type tiff

GIF

defaults write com.apple.screencapture type gif

Optional

killall SystemUIServer
defaults write com.apple.screencapture name "Screenshot"