add mimetype for compressed FictionBook2

https://bugs.freedesktop.org/show_bug.cgi?id=45795
Do not recognize all .asc files as application/pgp-encrypted

The change introduced in 2c3438c had the unfortunate effect of having all .asc
be identified as application/pgp-encrypted. As this should only happen when
we can identify armor headers, we now give a lower weight than text/plain to
the *.asc glob for OpenPGP MIME types.

https://bugs.freedesktop.org/show_bug.cgi?id=70539