I installed the addon on a Debian GNU/Linux machine, and it worked as designed.
However, then I deployed it to my colleague's Microsoft Windows machine, and there when one adds a filelink through this addon, the file is both uploaded to the NextCloud instance and also attached to the email (thus making a big email).
Thunderbird versions and addon config are identical on both machines.
I think that's most likely a bug in Thunderbird. There's nothing in the FileLink API that the plugin uses that can control that. There are a few other FileLink related bugs in TB as well.
I installed the addon on a Debian GNU/Linux machine, and it worked as designed. However, then I deployed it to my colleague's Microsoft Windows machine, and there when one adds a filelink through this addon, the file is both uploaded to the NextCloud instance and also attached to the email (thus making a big email).
Thunderbird versions and addon config are identical on both machines.