I know that it is sometimes necessary to send an attachment to an email that you received. With the iPhone 4, in order to include the attachment, you would "Forward" the email instead of "Reply" to it. You will then be able to send the attachment.
Here's a link to the manual of the iPhone from Apple's support page, so you, as well as other iPhone users, can review.
I trust this information is helpful for you. Have a good weekend!
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