You're not supposed to use Apple products for actual work.
Apple is for studs, Android for the geeks, and Windows tablets for getting things done
Seriously though, you can copy a link to a file in Dropbox and paste it into your reply.
If you want to attach an actual file, you need to send it from the program it was created in, and you can't attach it in a reply to a previous email.
I always thought that Android is full of great ideas hindered by half arsed software implementation and questionable hardware, while iOS is a very well executed poor design running on good hardware. After switching from an Android tablet to rMini, I see that I didn't realize just how limited iOS is. A list of things that simply can't be done is staggering, and many to me are just basics that I would never expect anyone would even think of omitting. I am still glad I switched, as I had bad luck with reliability of Android tablets, and I don't want to keep playing the lottery. But iOS has ways to go to catch up in terms of useability, luckily for Apple the majority of their customers buy it for glitter and brand name recognition and don't use it for complex tasks.