Starting in version 13, Thunderbird added support for online storage services through Filelink. It allows you to upload attachments to an online storage service and then replaces the attachment in the message with a link. Filelink can be configured to use many of the well known storage services out there and it can now be configured to support Synnefo deployments, too.
We created the ~okeanos filelink plugin for Thunderbird, to support our ~okeanos public cloud service. It uses the Astakos API for authenticating the user and the Pithos API for uploading and publishing the attachments. It is easy to use as it requires only the authentication token from the user. Also, it can be configured to support any Synnefo installation by specifying the correct Astakos and Pithos endpoint URLs.
Feel free to clone its code and create your own Thunderbird plugin by only changing the endpoint URLs. You can then use it with your Synnefo powered cloud service.