The cause of the blank icon is the lack of a file extension on the attachment (i.e. .pdf, .doc).

This typically occurs when you are using DMI (DayLite Mail Integration) and you've received emails with attachments from a classic Mac OS user. As you may recall, classic Mac OS doesn't use file extensions.

To get around this, have the user who is sending these emails attach the valid file extension to the filename before sending you the email.