Drag&Drop file attachments from Thunderbird - works for Slack, but not Discord?

I can drag&drop file attachments from Outlook into the Discord application (or into Slack or quite a few other programs).
I can also drag&drop files from virtually any other program into Discord.

I cannot drag&drop file attachments from Thunderbird into Discord, but I can do that into Slack.

What should I be looking for to get this working?

I'm running Windows 10 Pro.