In Mail, I'm guessing.
If the attachment is not one of the standard (supported) file types, you'll need to download and app that will open those file types first.
If the attachment has a dotted line around it, it has not been downloaded from the server yet. Tap it and it should show it's size and progress. It will have a solid boarder when downloaded (or in the case of pictures or PDFs, it may display inline).
Once the file is downloaded tap and hold for a popup menu. As long as you have a compatible app installed you should be able to select Open In… to copy and open the file in that app.