The file.php contains reference to filelib.php that lists MIME-types and their mappings to content-types. This does not include an entry for "ICO". Thus the following line should be added.
'ico' => array ('type'=>'image/x-icon', 'icon'=>'image.gif'),
Moreso, why are MIME entries even in a hard-coded PHP file? Apache maintains its own list of MIME associations in /conf/mime.types - and this includes ICO -> image/x-icon already!