If it is something other than a picture, you need an app that will handle that file type.
Tap and hold on the attachment in Mail for a grid of icons. Your choices, including compatible apps, will be shown. In iOS 5 or earlier you'll get a menu instead. The Open In option shows you the compatible apps.
Note: If the attachment is outlined with a gray dashed boarder it has not been downloaded yet. Tap it to start the download. When it is finished the boarder will be solid.
If you were talking about adding attachments to an email, most apps that create or store documents have an email option. Most of the time this is what you would use to send the attachment.