我正要放弃认为这是不可能的或至少是很容易的想法，然后我偶然发现wp_handle_upload_prefilter 过滤器，为您提供准确的要求！代码如下：

add_filter(\'wp_handle_upload_prefilter\', \'yoursite_wp_handle_upload_prefilter\');
function yoursite_wp_handle_upload_prefilter($file) {
  // This bit is for the flash uploader
  if ($file[\'type\']==\'application/octet-stream\' && isset($file[\'tmp_name\'])) {
    $file_size = getimagesize($file[\'tmp_name\']);
    if (isset($file_size[\'error\']) && $file_size[\'error\']!=0) {
      $file[\'error\'] = "Unexpected Error: {$file_size[\'error\']}";
      return $file;
    } else {
      $file[\'type\'] = $file_size[\'mime\'];
    }
  }
  list($category,$type) = explode(\'/\',$file[\'type\']);
  if (\'image\'!=$category || !in_array($type,array(\'jpg\',\'jpeg\',\'gif\',\'png\'))) {
    $file[\'error\'] = "Sorry, you can only upload a .GIF, a .JPG, or a .PNG image file.";
  } else if ($post_id = (isset($_REQUEST[\'post_id\']) ? $_REQUEST[\'post_id\'] : false)) {
    if (count(get_posts("post_type=attachment&post_parent={$post_id}"))>0)
      $file[\'error\'] = "Sorry, you cannot upload more than one (1) image.";
  }
  return $file;
}