In Outlook 2007 and above, the user has the ability to preview attachments without opening them in the program associated with them.
In some cases, the file can not be previewed.
If the following file types do not preview, do the following:
.pdf: Reinstall Adobe Reader and make sure it's the default PDF handler. It was likely installed before Outlook and didn't install the preview handler.
.csv: Run the PreviewHandlerEditor and add .csv files to be handled by the text preview handler.