If you tap on "download" in Mail, it won't be exported to another app.
It's now ready to view and export in Mail in the specific message.
If you tap on the attachment now you'll be able to view or save it.
After I downloaded the video I went to settings>general>usage and it took about 10MB of space. I deleted the email but It hasn't given back the 10MB yet even after turning off then on. So I'm guessing it's somewhere on the phone?
I use iFile on my jailbroken device to delete such files. Or install iTools on your mac or pc then scroll through the files to remove the ones occupying a lot of space. I dont know if the later will work on a device that's not jailbroken