Note that the "datas_fname" context allow you to give a specific
name to the document.

The previous method was not properly augmenting an attachment list, leading to
errors when using the "send by email" dialog box.

You can attach multiple documents as an attachment to the mail template.
Documents will not be regenerated should they have already been
generated beforehand and stored within a field of the object.

Note that now we can use this method to retrieve metadata from attachments.

Fix unit tests

Added the unit tests.
Added the translations.