Fix getting correct mimetype for "blob_plain" view for files which have
multiple extensions, e.g. foo.1.html; now only the last extension
is used to find mimetype.
Noticed by Martin Waitz.
Signed-off-by: Jakub Narebski <jnareb@gmail.com> Signed-off-by: Junio C Hamano <junkio@cox.net>