You need to think differently when using iOS, go to the app with the attachment and open it, you should then see a small box this a curved arrow in it. Touch this and you should get the option to email (if you tell us which app specifically we can tell/show you where all this is).
Another option, if the app supports it, is to save the attachment in Dropbox. You then create a link and copy/paste the link into your email.
The Archangel