Hello and thank you for responding.
I only use the iCloud mail service and not the Mail app. I've never been happy with the Mail app itself and was very satisfied using iCloud for my email.
When this started happening, somewhere around 6-8 months ago, I immediately checked the status of iCloud Mail and found no problems listed at all. It only seems to happen with attachments that are above 1mb in size. They can be JPEG, PDF, EPS or just about any other type of file and the problem pesists. I click on the icon for the file and it's totally uncertain if the file will download. When I click the icon on large attachments, absolutely nothing happens at first. Sometimes nothing ever happens. It's as though I didn't click anything at all. Other times, often when I've clicked the attachment icon 5 or more times, the "file" will fly down to my "Downloads" folder.
Just yesterday, I double clicked (twice) on a 2.5mb attachment file (a docx file) and it downloaded - after about a 20 second delay.
Today however, I tried the double click (twice) on a 7.9mb EPS file and absolutely nothing happened. Nothing at all.
As a work-around, I've learned that I can forward the email and its attachment to my work email (we use Microsoft Office 365) and I was able to download the 7.9mb EPS file instantaneously.
To say this is frustrating is an understatement. The few settings I can find in iCloud Mail do nothing to assist. If there is a setting in the OS itself that I can try I will but I've found nothing. I am nowhere near my iCloud storage limit (I have over 27GB available)
Thank you and I hope you or someone can find a solution.