Extra support for some archive types

[Ralph Seichter]

* korsar:

> 1) ACE has no support at all, need to add it

Not having seen ACE archives in the wild for more than 10 years, I don't
think so. Besides, to my knowledge ACE is a proprietary format with a
Windows focus and a license that does not permit including it in free
software.

> 2) gzip - gzip -d command changes original file names of extracted
> files, [...] 7z, please.

You can adjust filetype-to-decoder mappings in your amavisd.conf if you
don't like the defaults.

-Ralph