Prova questo:
if ($_FILES["photo"]["type"] == "image/gif" ||
$_FILES["photo"]["type"] == "image/jpeg" ||
$_FILES["photo"]["type"] == "image/pjpeg" ||
$_FILES["photo"]["type"] == "application/zip" ||
$_FILES["photo"]["type"] == "application/pdf" ||
$_FILES["photo"]["type"] == "application/msword" ||
$_FILES["photo"]["type"] == "application/vnd.openxmlformats-officedocument.wordprocessingml.document")
Puoi guardare i diversi tipi MIME qui:http://en.wikipedia.org/wiki/ Internet_media_type
