images attachments cannot be mapped correctly to image fields

Hi,
I recently updated Mailhandler from "7.x-2.x-dev, 2012-09-10" to "7.x-2.6".

Now, when imported emails contain images as attachment they cannot be mapped correctly to the image field of my content type.

On import, the images are correctly saved to the "public://mailhandler_temp" directory. But they stays there and then, in the "Recent log messages", I got notices of this kind: "Unable to generate the derived image located at public://styles/thumbnail/public/pictures/flitter/sinularia-ouvert.jpg.". The result is that my content shows the message sent by email, but not the images.

This was not happening with the "7.x-2.x-dev, 2012-09-10" version (attached is a copy of the working version I have on my production site). This version was patched with "mailhandler-multiple-email-1921318.patch". No other modules were updated and if I revert to the "7.x-2.x-dev, 2012-09-10" version, everythink works. So I don't think this is a duplicate of https://drupal.org/node/1843168.

Drupal Core version: 7.22
Version of Feeds module: 7.x-2.0-alpha8
Version of Multiple email: 7.x-1.0-beta1

Thanks for your help.
Best regards.