thumbnails not included for attachment fix

Even safer is to use wp_upload_dir();

function query_images() {
    $sizes = array('thumbnail', 'medium', 'large', 'full');
    if (function_exists('get_intermediate_image_sizes')) {
      $sizes = array_merge(array('full'), get_intermediate_image_sizes());
    }
    $this->images = array();
    $upload_dir = wp_upload_dir();
    foreach ($sizes as $size) {
      list($url, $width, $height) = wp_get_attachment_image_src($this->id, $size);
      $filename = str_replace( $upload_dir['baseurl'], $upload_dir['basedir'], $url);
      if (file_exists($filename)) {
        list($measured_width, $measured_height) = getimagesize($filename);
        if ($measured_width == $width &&
            $measured_height == $height) {
          $this->images[$size] = (object) array(
            'url' => $url,
            'width' => $width,
            'height' => $height
          );
        }
      }
    }
  }

using site_url() (not up_upload_dir(), unfortunately) worked for me — thanks!

I had the same issue – and had to revert to older code that doesn’t check file paths & image sizes – just builds a list of images base on WordPress’s wp_get_attachment_image_src()

Before making changes to the plugin code, check to see if you have jetpack>photon enabled. It rewrites the thumbnail urls to include the wp.com cdn in such a way that that makes the fileexists check fail.
I disabled photon and the thumbnail urls reappeared in the JSON feed.