JPEG image attachment on calendar entry not accessible using iOS 12 calendar app

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Running Nextcloud 14.0.4, I’ve used emClient 7.2.33834.0 on Mac OS X 10.14.2 to attach a JPEG image to a calendar item.

Still on Mac OS X 10.14.2, Apple’s Calendar application Version 11.0 (2245) can successfully fetch and display this image.

However, on iOS 12.1.2, the Calendar app does not successfully fetch and display the image. The error, “Cannot Open Attachment” is displayed when tapping the image.

Is there any reason the iOS calendar app cannot display the image, while the Mac OS X calendar application can?

I don’t see any messages (error, warning, info, or debug) in the logging which appear relevent.