This has a long history: https://bugzilla.mozilla.org/show_bug.cgi?id=168680

If you right-click in the event Attachments box, a URL can be inserted, and according to a recent discussion, the same can be done with local files.

Many thanks for this.

Actually, having looked at this again, the weblink option is also blanked out when I right click on the attachment box. So even this option isn't available!

What version of TB and Lightning do you have? See the picture for the dialogue that appears when you right-click in the Attachments box.

The box is blanked out. I have the latest version of thunderbird. See picture. Thanks.

Is the calendar in question a local file or a web-based, e.g. Google, calendar? I believe attachments are supported in the former but not the latter. If it's a local calendar, do you have any other calendar-related add-ons?

Hiya - thanks for your input. The calendar is a google calendar linked into Thunderbird through two add-ons: 1. Lightning 2. Provider for Google calendar

I disabled, restarted and then enabled these add-ons, but the problem is still persisting.

I think you've answered your question - attachments aren't available for online calendars.

So the attachment is only available if the calendar doesn't synchronise to an online calander?

I can't locate the reference at the moment, but yes, that is my understanding. If you create a local calendar, you should see the attachment option. A post from 4 years ago suggests there are additional options if you set up Filelink, but I suspect that doesn't extend to online calendars.

Ok - thanks for this. ben