@jammerdan Have you mentioned a Filelink add-on for Thunderbird? I don’t think so, so let me suggest that
Many email servers do not accept messages with large file attachments. The file size restriction varies depending on the mail server configuration. While you might be able to send a message with a large attachment, the receiving mail server might refuse to accept a message with a large attachment. This is all beyond the control of Thunderbird.
Thunderbird Filelink eliminates this problem by providing support for online storage services. It allows you to upload attachments to an online storage service and then replaces the attachment in the message with a link. The message recipient clicks the link to download the attachment. As additional benefits, sending and receiving large files is much faster and both you and the recipient will save disk space.
The Thunderbird development team seems to recognize the potential issue related to privacy of uploaded attachments:
Q: Can the storage service view my attachments?
A: Unless you encrypt the file before uploading, the storage services will be able to view the file, as will anyone who obtains the link to the attachment. Users must decide on their own which service provider they trust with that responsibility.
The issue can be avoided with Tardigrade since uplink encrypts the file before uploading.
There are Filelink add-ons available for Dropbox, Box.com, Nextcloud, etc.
Currently you need to share the file via uplink command line to copy and paste the link to a message, which can be avoided with the add-on for Tardigrade.