Remove email attachments from iPhone
The attachments will not remain on the phone past the point that the email itself is in the default number downloaded by Mail (on my iPhone it's 50 most recent messages). Once the message falls out of that, and/or is deleted, its attachments should be removed from the phone memory. 

The other way to be entirely sure they are gone is to disable 'Mail' in settings for the account you are using, wait a couple of seconds (let it do its thing) and re-enable. This will purge everything related to that mail account and let you redownload just the messages. If you download a lot of attachments versus the amount of mail you receive, it might be worth doing this on a weekly-or-so basis.