If mod_mime_magic does not know the content-type, do not attempt to guess.

PR: 16908
Submitted by:   Andrew Gapon <agapon@telcordia.com>
Reviewed by:    Justin Erenkrantz, nd, jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@99977 13f79535-47bb-0310-9956-ffa450edef68