so you are worried about having every attachment download automatically right? and not having your e-mail pushed to your device, correct?
what email app are you using? stock? 3rd party?
i am using stock and i do not have this problem. i just looked through the settings of the stock mail app and found nothing to turn on/off attachment downloading.
to not have anything show up in the gallery, use a file explorer to place a file called .nomedia into the "download" folder. this will cause the gallery to skip over that folder.