Hello !
A new thunderbird extension is available at Seafile Filelink2 :: Modules pour Thunderbird 
You can find the source code at Github seafile thunderbird 
I need your reviews but the plugin is still experimental so be nice 
and report issues on github.
Bastiencyr
Hello @Hannes_Hartung,
You said in another post :
Unfortunately, I have not yet managed to get your new plugin to run. I use Thunderbird in the current version in a portable installation. I can configure my Seafile server after installing your plugin in Thunderbird, but when I leave this dialog and open it again, the data I entered regarding my Seafile server is no longer available.
It’s a bug you’re right but actually the settings are just gone for you. They are just not loaded. Create an email and then create a link to seafile by adding an attachments, it will works well.
Hallo,
indeed, now the settings remain after I’ve saved them. However, when I now write an email, attach a file and select ‘Seafile’ the error message:
‘An unknown error occurred while exchanging data with Seafile.’
The attachment is not uploaded to the Seafile server, no link is presented in the email.
I have to confess that I still use the last available windows version for Seafile server. The error message appear if I go directly via ‘http’ and port 8000 or by using a ‘https’ reverse proxy server (IIS). Thunderbird & Seafile server are running on different machines in my local network.
As an alternative I have Nextcloud installed in a VM and the corresponding filelink plugin works fine.
Greetings & again thanks for your work on the plugin!
Hannes
I catch almost all errors. I need to understand better your setup and check your logs. Can you open an issue on Github and continue the discussion here : https://github.com/bastiencyr/seafile-thunderbird-filelink ?