There is a jailbreak app to download attachments to the phone within the mail app, but so far I don't think there is anything that lets you send one within mobile mail. But who knows?!?!? Maybe someone will read these posts and have a lightbulb moment?
Well for example in the photos app you attach photos to an email. And if you have an app like pages or docs to go you email them from the specific app itself. You need to add the attachment not from the email app but from the app that has the attachment you want.
Press down inside the text field like you would for copy/paste. Then when the copy/paste menu pops up, click the right arrow for more options. Insert should be listed there. May be another way to do it, but that's what I have found so far. Quite handy.