There are two groups of mail attachments:
Ordinary attachments, which are bound to their applications. You open them (and their applications) directly from the Attachments pane. For example, Calendar Manager appointments, mail messages, icons and images, voicemail, and executable files.
Special attachments, which are not bound to their application. You cannot open these files directly from the Attachments pane. For example, specially-formatted text files created with desktop publishing software.
If there are more attachments than can be displayed in one row, you need to scroll down with the Attachments pane scrollbar to see all the files.