To be able to open an email attachment you need to have a compatible app available on the iPad. So, for example, if someone sends you an email with a .doc or .docx (Microsoft Word) attachment, you'll need to have an app on your iPad such as Apple's Pages app, that can 'open' Word formatting documents.
If you have an app that's compatible with the attachment then, when you touch the attachment it will either complete the download if it's a large attachment and/or open it in preview mode. When you touch the previewed image, if you have a compatible app to process it, you'll see - in the top menu bar - a little curved arrow icon. Touch that and you'll be offered a 'Open with' choice and it will then list the app(s) that can open that document. Select the one you want to use and you'll be transferred to that app and a copy of the attachment will be moved so that the app has a local copy - even if the email is deleted.
Tim
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