
Nowadays, file extensions are not really used by the computer — they're used by humans who want to know the file format. It's a buggy system, since the extensions can be wrong. The Word document can have .txt extension, the .html file can be a JPEG.
When you are using a GUI, you can have icons that do the same job, but you are not always using a GUI; the GUI often just repeats what the extension claims without checking; and the GUI might not even understand the proprietary format.
But personally, I'll keep using extensions. I'd rather have some indication of what the file format is, and hope it was set properly, rather than having to guess every time.