While I was using this module together with Insert I noticed that the link for a image named for example: 'Selección_016.png' (please not the: ó)
Was being inserted with the following path:
http://example.com/sites/default/files/Selecci%C3%B3n_016.png
And the filter module was filtering out the image.
It seems that @getimagesize($local_dir . drupal_substr($src, $base_path_length)) is not able to open the file with that kind of encoded url.