After updating Usermin from 1.020 to the latest 1.270, some kind of attachements are not visible or available when choosing an email message with attachments. I don't know for sure what kind of attachments that are, but it looks like most of them come from Microsoft Outlook products and email generating tools.
Also - when View as HTML is selected as default, some emails looks like empty until View as text is clicked.
I and many of the 2000 users on our server wish this could be like it was in the 1.020 version of Usermin. There everything worked just fine :-)