You may experience any of the following issues:
These issues are typically caused by the Google Photos app being set as the default photo viewer. Follow these steps to reset the default photo viewer to your phone's Gallery app:
If the issue persists, it is aslo likely caused by a network issue when downloading the attachment. Check that you are connected to a network (see I'm having problems connecting to a Wi-Fi network), or try to download the attachment later.
NOTE: On newer HTC devices running on Sense 7 onwards, the Gallery app is replaced with Google Photos.