It's difficult to add an attachment when replying to an email. I use a truly awful workaround. I don't use the particular app you mention but, if you can email a file from there, then you can copy-and-paste the email that you want to reply to into the message field after you've created the blank email from within your WP app. Then you have to add the To: and other addresses manually. I know of no other way because, from within the iPad's native Mail app, there's no way of adding an attachment. It's not really Mail's fault - it's because files in iOS are isolated ('sandboxed') in their app's private filespace and that is not visible to other apps. So Mail simply can't access the filespace of that WP app. It's only from *within* the WP app itself that the file can be accessed and attached to a (blank) email.
Tim