I've just been trying to create a .crt file, so I can copy the contents (text) from a web page in to it, but Mac OS doesn't seem to want to let me do it easily.
Normally in Windows I would right click > new file, or even open notepad and save as (as it lets you save with any extension), but TextEdit in mac forces me to save as RTF, and I seem to be unable to rename it in finder afterwards?!
I ended up opening the terminal and executing
touch cert.crt just to create the file. I need to explain this process to some other users - there must be a simpler way.