I use cmd-shift-4 to take screenshots on the mac and save to disk, and cmd-ctrl-shift-4 to take screenshots and copy to clipboard.
When I want to put these into, say, mail.app, I notice that the extensions are different. The clipboard images are in tiff format and the saved images are in png. I know I can open them in preview and change the type - how do I do this so it saves ALL images as pngs by default?

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