How to open mail with attachment?

It depends what the attachment is. Opening the actual textual content of the email should be handled by the iPad's Mail app. Mail can also open (display) certain attachments too, such as PDF and Word, but not all. For others you'll need a compatible app. If you're using webmail and not the iPad's native Mail app, then Safari can only download a very restricted range of attachments. For others, you'll need to use a third-party browser, such as iCab.

Tim