jka-compr.el doesn't recognise gzipped files from their magic bytes

[Chris Moore]

I'm using an application which saves natively in .sif format, or
optionally in .sifz format, which is the same as .sif, only gzipped.

Opening a .sifz file in Emacs shows binary junk - Emacs doesn't
recognise that it's really a gzipped file, even though it does know
the 'magic' string that identifies all .gz files (see the value of
jka-compr-compression-info-list).

So shouldn't jka-compr look for the magic strings, and identify
gzipped files that way?

I notice that '.svgz', the common name for gzipped .svg vector
graphics files has been hardcoded into jka-cmpr-hook.el, rather than
adding general support for gzipped files, whatever the extension.