At the moment, no attachments are displayed for sent emails, even if they have been sent.

That's really stupid and I think it's simply a bug, but support said it was “not a bug, it was a design.” Whatever: You must be able to see what you've sent, including attachments!

Otherwise, you have to go out into the external mail program to check whether the attachment has been sent along with it, and it is precisely this tool change and the manual check that is completely against the ClickUp approach.