Unable to hide file extensions in El Capitan

I can’t hide file extensions on some files in El Capitan. When I go to 'Get info' on certain files, the option is greyed out. And is there a way to hide all file extensions at once instead of doing this file by file?

I believe Deeper for Mac has been updated for El Capitan, and IIRC that's the sort of thing it can do.

So why does the option to show all filename extensions in Finder Preferences>Advanced only hide SOME filename extensions when unchecked, but not all?

thanx Chas_m but I can't find any option that allows me to hide extensions.

D'oh! I had totally forgotten about the Finder Preferences>Advanced/> Show all filename extensions'. Normally after reformatting my OS, this is one of my 'must do' checkpoints but must've been distracted at the time. Thanx again chas_m and to etmarcr for reminding me.