Email attachments not downloading properly in Mac Mail - Mojave and High Sierra

Sorry if the use of the term hypothetical offends. If it is an issue with Mail seeing these attachments as infected, then the attachments are apparently being examined ONLY when their source email is processed by a Mail rule AND that rule attempts to move the email to a different account. By "hypothetical", I only meant that there was no direct proof that this is indeed the cause of this issue, although it is a possibility. My reasoning is that I doubt that 100% of the attachments I received (that were subsequently moved to another mailbox by a Mail Rule) were infected, as ALL of them became inaccessible. They came from so many sources, and many I sent to myself from other secure systems. Some were photographs resent from a different account on my iPhone. The only thing that they had in common is that they were received in an account (IMAP) residing on a server managed by an organisation that would have made international news if they were consistently passing on infected attachments, or attachments modified in some manner so as to appear infected. In the cases where I was able to locate an original digital copy of a few of these photos, sent them to myself using an account that was not processed by Mail rules, then the attachments (photos) came through unscathed.

One of the interesting things I've uncovered: If I turn off Rules, and then look at the attachments on emails that WOULD have been moved by a Rule, I can see that the attachment is fine. Now, if I then turn on Rules again, and THEN Apply Rules, the email is moved to the correct destination, BUT, the attachment is still OK. So the attachments only become inaccessible if their email is moved by a Rule before the attachments are viewed. Viewing before the Rules process an email apparently completes some process that allows the attachment to remain intact. At least, that is what I am seeing as of testing done today.

As for now, I have developed my own procedures to ensure I do not lose any more attachments. I can say that I am extremely disappointed in Apple. If this is actually a bug, there is no good explanation as to why it has not been fixed. If it is expected behavior, then Apple definitely owes their loyal users an explanation for the behavior AND a workaround for people to avoid losing their valuable work product!

Please note that I rarely receive attachments on my iCloud (.mac) account, and so have not, to my knowledge, lost any attachments from that account.

I read the hypothetical reason - that Mail sees the attachments in question as possibly malware or carrying malware. That is, I suppose, a reasonable assumption. However, in my case, 100% of the attachments received and moved to a local Mail folder (mailbox) are unretrievable. JPG, PJEG, MOV, MP4, MP3, PNG, PDF, are just a few of the file types (but not all) affected. The source, that is the person or entity sending the email, is irrelevant, as well. ALL attachments from all sources are affected. I tried to create a rule to stop rule-processing when there is an attachment, but there does not seem to be a good way to do it. Instead, I have added a rule at the beginning of my rule list that stops ALL rules. Then, after I see if I have attachments that need to be saved and do so, I can turn off THAT rule and allow the remaining rules to run. It works, but is cumbersome - a total PITA.

I had this issue today, and my work around was to open up a webpage, go to iCloud.com and sign in there. From there I opened up my mail and my attachment showed up as a download link that I was able to click, download, and then open up. It's an extra step, but it worked for me and I hope it helps until they get to it.

Best wishes

Apple hasn't answered the questions. It is users like you and me that answers them. I have found the answer to why it happens but not how to get rid of it. See here: https://discussions.apple.com/thread/250017426?answerId=250287998022&page=1

I have never had problems with downloading any attachments using Mail.

Look at this possible solution by babowa, it just may be that the problem lies upstream.

Attachment to Email Stuck Downloading - Apple Community

What is hypothetical about it? I read about it and then when I checked my computer using Onyx I got the answer. Mail thinks some attachments might have malware. I haven't found how to get around it. Only way for me is to maually download the attachments from my mail server.