Always use lowercase for filenames. While you don’t have to do this, Linux based servers treat uppercase letters and lower case letters as different. Meaning zombie.html and Zombie.html would be treated as completely different files. Windows servers ignore case. If you ever need to switch from windows to Linux (or you’re handing a site off to a client or similar) a lot of headaches and random errors where files can’t be found can be avoided if you just use lowercase consistently everywhere.
Getting Braaains Should Be Fun