I know some coding are not case-sensitive; I did not know about the file extensions being case-sensitive.
It’s probably the environment in this case. Windows isn’t case-sensitive, so you’ll work on your web pages on your home computer (running Windows 7/8/10, whatever) and it doesn’t matter if you mix up the letter-case – paths will keep on working. But then you upload your files to your web host, and they’re very likely running a UNIX-based system (UNIX is free, while Windows incurs licensing so costs, so UNIX-based web hosting is very common). Unfortunately, this is when paths break because UNIX is case-sensitive.
Getting into a habit of using only lower-case letters to name your files will save you many headaches in the future. Also quicker to type since you’re not reaching for the Shift key all the time :)