i just got the iPhone 5 and connected my Gmail account to the mail app. however Verizon charges through the nose for data, and i realized that the most data that i chew up is through email attachments. this would normally not be a problem, but the iphone seems to download attachments automatically! i know this because i opened an email with a JPEG attached, and then set the phone on airplane mode. after that i was still able to open the photo from the email app when i went back in, so i know it downloaded to my phone somehow. is there any way to stop this somehow, and save myself the data charges?
About the only setting you can change that pertains to this is the Load Remote Images toggle that is found in the Settings app under Mail, Contacts, Calendars. Most important thing, still, is to make sure that is off.
This may or may not do the trick for you. I get the feeling this switch was designed more for the retail emails that are sent out - full HTML with tons of images.
Have you performed your test with a PDF file? By default iOS creates an icon for you to tap before it downloads the attachment. It is possible this switch is on, and it is downloading images.
If the afore-mentioned ideas don't work, would it be possible to not open emails that have the paperclip icon next to the Subject/Sender, and instead wait until you were on a wifi? This could protect from data usage, however, if you must receive large quantities of large attachments for a specific purpose this may not be practical.