Remote images (attachments) are automatically loaded when you read your emails through the Mail App. But if you are short on data, do as follows to stop loading images in Mail app on iPhone automatically:
1. Open the Settings app
2. Tap to open the Mail, Contacts, Calendars settings
3. Scroll down to the list of Mail settings and make sure the Load Remote Images option is toggled OFF.
4. From now on, you get to decide which images should be loaded in your emails and which images should not be shown.