It's not that Netscape is finicky, Netscape is actually the smarter of the two. All browsers except for Internet Explorer pay close attention to the mime-type of a file. This is mainly to protect your computer from dangerous file types that could contain viruses sent with an incorrect mime-type. Internet Explorer, being a true blue Windows application, only cares about the file extension. It will load the file into memory, even if the file is actually an executable file with a .css extension. It might throw out an error if the file isn't what it expected, but often that's all it takes for a virus to gain a foothold in your machine.
So, browsers paying close attention to mime types is a security feature, not them being finicky.