Mail Attachments not downloading

For example I receive an email with GP CHALLENGE in the title and under Rule 2 in the tab of the preferences window this is put into a special mailbox. Unfortunately any attachment is shown with the word "downloading" in blue underneath.
On the internet I have found only one article on the subject advising that this was a problem in High Sierra. I did not notice it then but I get very few attachments with those emails that are processed by the rules.
Has anyone experienced this with Mojave?
(Try it. Make a rule for moving incoming mail into a mailbox then send yourself an email with an attachment)