detect: fix fileext and filename negated match

detect: fix fileext and filename negated match

fix bug in fileext and filename preventing negated match to work
correctly. Previously, negated fileext (such as !"php") would cause a
match anyway on files that have extension php, as the last if would not
be accessed.

Using the same workflow as detect-filemagic we remove the final
isolated if and set it as a branch of the previous if.