mime-attachment

Yes, many file types cannot be opened on a mobile device, especially iPhones. However, a .jpg should easily open.

I'm not sure, this may be a problem with the device, when researching this nothing came up so you may one of few with such a problem. The last question on Apple Discussions was from five years ago (2015), so I would recommend going to a local Apple store about this.

Can you show a screenshot of the screen that says "Cannot Download Attachment"?

That is definitely interesting. Maybe try using a computer to view them. Or simply try again later, this could be a temporary problem with the application.

Have you tried restarting your phone?

I would also recommend (if everything else fails) to ask the person who sent the attachment to you to send the file in a different format (either saving the file under a different file extension if it's local or put the original image through an online image converter).

Can you please elaborate? By "straight jpg download" I would assume this is an image? What are you attempting to open from (phone, computer, etc.)? What happens when attempting to download the file?

You are very welcome! I had fun attempting to troubleshoot this! Please, let me know if the different formats thing works! :)