[Grhm]

I just tried it for myself, and you are right, if you send yourself an HTML file and open it in the Fastmail web interface, the links don't work!
I suspect this is a bug in the code that handles attachments in the new user interface.
If I'm right, the good news is that it only affects recipents who are using Fastmail - the vast majority of people you are contacting will still be able to follow the links as before.
I suggest you open a support ticket.


A work-around: Instead of opening the attachment directly in the e-mail, click on the check box beside it and select 'save to file storage'. Then go to file storage and open it there. The links will then work fine.