Hello LAV2,
To help isolate this issue, let's have you download these attachments at iCloud.com. Here are the steps to do so:
- In Mail on iCloud.com, open a message that contains an attachment.
- Click the file or image you want to download to your computer. The file or image is downloaded to the location specified in your web browser’s settings.
If the attachment opens as expected, this would indicate an issue with the mail client on your computer, and it would be best to try quitting, and reopening the app. Then, if needed, restart your computer to see if that helps.
You can read more about the process of opening attachments at iCloud.com here:
Download email attachments from Mail on iCloud.com
Best Regards.