Adding attachments to an open HP Support case

I don't think this is documented anywhere. But I just learned how to add updates to the HP case-file. Including attachments.

To: support_am@hp.com
Subject:

CASE_ID_NUM: [case number, such as 36005555555]
MESSAGE: [text]

Any attachments to it will be automatically imported into the case. LOOKING at the case itself is a lot more complicated, and I'm still not sure of the steps. But this should be of use to some of you.