gitweb: Fix mimetype_guess_file for files with multiple extensions

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>

gitweb/gitweb.perl

@@ -1197,7 +1197,7 @@ sub mimetype_guess_file {
 	}
 	close(MIME);
 
-	$filename =~ /\.(.*?)$/;
+	$filename =~ /\.([^.]*)$/;
 	return $mimemap{$1};
 }