A message you open in Lotus Notes has blank body, and the content is attached to the message in a temporary attachment (i.e. attxxxxx.dat) due to the message header not having the "Content-Type" set.
This issue has been reported to Quality Engineering under APAR LO66887 and SPR MCHZ8QVTHU.
Diagnosing the problem
View the message Page Source (Open the message, go to View -> Show -> Page Source) and check if "Content-Type" is blank
Resolving the problem
This issue was reported to Quality Engineering under Software Problem Report (SPR) MCHZ8QVTHU. When the "Content-Type" is blank, then it can be assumed the message is in Plain Text format. As a workaround, double click on the attachment and select View.