The text “Missing Plug-in” or a blue brick is displayed instead of an attachment.
This happens randomly since Mac OS X 10.6.x, and it is not caused by Attachment Tamer. There does not seem anything in particular that triggers the behavior, but it at least does not seem to affect the actual content of the message: the attachment is still there.
Exceptionally, this may also happen for a good reason: when the message is corrupted or the attachments have been removed from the message.
Workaround: There is no reliable workaround. Try reinserting the attachment, closing and reopening the message, quitting and relaunching Mail. Note that the issue is not related to Attachment Tamer, so removing Attachment Tamer will not help.