bug#52477: 27.1; .crx and .crx3 are zip files

[Eli Zaretskii]

> From: Michael Albinus <michael.albinus@gmx.de>
> Date: Thu, 16 Dec 2021 09:56:57 +0100
> Cc: "Trent W. Buck" <trentbuck@gmail.com>, 52477@debbugs.gnu.org
> 
> > The first couple of bytes of that files is "Cr24", while a normal zip
> > file starts with "PK".
> 
> Yes, and that's why libarchive(3) doesn't handle it. Sadly, I cannot add
> it to tramp-archive.el, therefore.
> 
> But there's no reason to prevent it from adding to arc-mode.el and/or
> dired-aux.el.

If the file doesn't follow the PKZip spec in the initial signature,
who knows what else in that spec doesn't it follow?  The ZIP spec has
a lot of special rules and formatting requirements, so if a file
doesn't follow them, how can we support it without knowing its exact
format specification?