Thunderbird can be set up to automatically use Nextcloud to send large attachments. This is convenient because not all mailservers have the same maximum attachment size. I’m mostly documenting this because I keep forgetting about it.

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Step 0-a: install Thunderbird and set up your mail account.
Step 0-b: if you don’t have Nextcloud server, install one. I think this will work with ownCloud as well but I haven’t tested it. Here are my own thoughts about setting up Nextcloud on Debian. You can ignore the Active Directory part if you don’t need that.

Step 1: install the Filelink plugin
In Thunderbird, press Alt to bring up the menu bar if it isn’t shown and from the Tools menu choose Add-ons.

In the left bar select Extensions and search for Nextcloud. Install the Nextcloud for Filelink extension.

After the installation Thunderbird wants to restart.

Step 2: configure the Filelink plugin
Press Alt to bring up the menu bar, select Tools > Options.

In the Attachments section select the Outgoing tab. Check Offer to share for files larger than 5 MB. Select your size preference. Then click Add.

In the Set up Filelink dialog box select Nextcloud.

Provider Just enter a name here, does not have to the URL
Server URL Your Nextcloud server’s URL
Port number Your Nextcloud server’s port number
Username Your Nextcloud account’s username
Storage path in Nextcloud /Mail-attachments – or any other folder you like. I suggest you exclude this from synchronization on your local clients; you already have the files you mailed.
Password for uploaded files OPTIONAL: this is NOT your Nextcloud account’s password. It is the password the recipients need to download the file. You cannot change this afterward and it is for all linked attachments.

Click Set up Account.

Now enter your Nextcloud account’s password. I suggest to you check the Use Password Manager to remember this password box but that’s up to you. Click Ok.

If all is well your account is now set up.

Test it by attaching a file that’s larger than the threshold you set. You should get a warning from Filelink. Click the Link button.

The file is being uploaded.

Once uploading is complete a message will appear in the mail.

The recipient can click the link in the mail.

The recipient is presented with a download dialog box.