经过一些搜索，我找到了所需的参数：https://gist.github.com/luetkemj/2023628 （第231行）

//////Search Parameter
//http://codex.wordpress.org/Class_Reference/WP_Query#Search_Parameter
\'s\' => $s,                              //(string) - Passes along the query string variable from a search. For example usage see: http://www.wprecipes.com/how-to-display-the-number-of-results-in-wordpress-search 
\'exact\' => true,                        //(bool) - flag to make it only match whole titles/posts - Default value is false. For more information see: https://gist.github.com/2023628#gistcomment-285118
\'sentence\' => true,                     //(bool) - flag to make it do a phrase search - Default value is false. For more information see: https://gist.github.com/2023628#gistcomment-285118

我的代码现在看起来像：

$args = array(
    \'paged\' => $paged,
    \'posts_per_page\' => 9,
    \'post_type\' => \'attachment\',
    \'s\' => $s
);
query_posts($args);