Let's take a text editor

And save the text with the proper suffix : .html

Other suffixes

Type	Suffix
Images	.jpeg, .gif, png
Video	.mov, .mpeg
Sound	.mp3, .wav, .aiff
Text	.txt
HTML generated by a script	.pl .cgi .php, .asp

MIME (Multipurpose Internet Mail Extensions) types

When receiving a document from a server, the browser doesn't initially look at the suffix : it receives from the server a header that informs it about the document : its type, size, age

About .htm