In message , Wolf K
writes:
On 2017-12-14 00:24, Your Name wrote:
On 2017-12-14 03:16:11 +0000, Wolf K said:

On 2017-12-13 19:37, Your Name wrote:
[...]
* ... you can't rely on the OS to do that since a JPEG image file can
actually be opened in a text editor as the file's data, even if
it's rarely useful to do so.

That's what Open With is for.
Open With is near useless if you don't know what the file actually
is. You'd have to Open With with every app you have until you found
one that could open it properly.

If we're talking about user convenience, I agree, showing a file's type
as part of the filename is very useful. (But IMO a three-letter
extension is too limited). There are many other useful conventions, eg,
in icon design. These are converging on a common standard.

How about this thought, to add fuel to the fire (stir, stir ...): the
filename extension _is_ metadata. Not as intimately bound as metadata
_within_ the file (because it _can_ be renamed), but arguably more
closely bound than metadata in an attached spoon*. [As another has
pointed out, they're not limited to three characters - though I would
say three _is_ enough to allow an awful lot of filetypes!]

If we're talking about choosing a program to open a file, extenions
aren't needed. It would be easy to ensure that Open With offers only
programs that can open a given file without reference to an extension.
Just standardise metadata (eg, as a series of slots, some which must be
filled, others for dev or user options). Easy peasy.

"Just" standardise metadata. Good luck with that. (Plus - even if you
did succeed - with converting all the already-existing files.)

Have a good day,

* Oh, all right, fork.
--
J. P. Gilliver. UMRA: 1960/1985 MB++G()AL-IS-Ch++(p)Ar@T+H+Sh0!:`)DNAf

"Bother," said Pooh, as Windows crashed into piglet.