OSX Screenshot App: Can't change screenshot file format I can't get the MacOS screenshot app to change the file format.
I've tried to change the settings with

$ rm  ~/Library/Preferences/com.apple.screencapture.plist
$ defaults write com.apple.screencapture type -string "png"
$ defaults write com.apple.screencapture location "/tmp/"


rebooted, just in case, but when trying to save a new screenshot, I still can only save it as a .TIFF-image.

Some additional info: This issue was present even before the update from Mavericks to Yosemite. 
killall SystemUIServer does not make any difference. 

Interestingly: 

$ defaults read com.apple.screencapture
{
    location = "/tmp/";
    type = png;
}


File permissions, numeric UID and GID: 

$ ls -lacn  ~/Library/Preferences/com.apple.screencapture.plist
-rw------- 1 501 20 74 Okt 31 15:59 /Users/jottr/Library/Preferences/com.apple.screencapture.plist


also, 

$ ls /Library/Preferences/com.apple.screencapture.plist
ls: cannot access /Library/Preferences/com.apple.screencapture.plist: No such file or directory


So there's nothing in /Library/Preferences/ that could override the settings in ~/Library/Preferences/.

OS X 10.10 (14A389)

$ echo $OSTYPE
darwin14.0.0


Screenshot App Version: 1.8 (117)

I checked the File Permissions with diskutil, no errors.
Are you using ⌘ Command+⇧ Shift+3 and ⌘ Command+⇧ Shift+4 instead of Grab.app? Grab produces a .tiff file, but the keyboard shortcuts will produce whatever is set in that defaults entry (by default, .png).