File Types

  A file type may be determined by extension (e.g., .txt),
a general name (e.g., archive), or a hint (e.g., .text).
Certain extensions are so predominant that they may safely
be used to identify files (although they do not in fact
provide any such guarantee, except according to sense).

.txt Visible ASCII text.
.text Indicates a formatted kind of .txt.
.html HyperText Markup Language.

.jpg Most common extension for JPEG images.
.jpeg It is silly to use this one.
.gif Classic lossless web image format (a few colors).
.png Portable Network Graphics (smarter than GIF).

.gz Gzip'ed contents.
.zip Zip archive.

dotfile  Convenient to be kept hidden (begins with a .).