And the right click on the iPad is where, Gandalph?
aronchick: so you're saying you haven't had this problem before now? If that's the case, then it's likely to be an issue with the attachment stored on the cloud server (your ISP or email provider) and likely temporary. If you've moved or deleted attachments on another machine that could also cause the issue you're seeing.
@OP: Was the attachment you were trying to open already downloaded or did you get that error while downloading it before opening it? If the former, then it might be the attachment itself, if the latter, as chas_m said, it might be a server issue accessing the attachment..
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