Outlook 2010 has a feature that allows you to preview attachments and documents if the handler is available for that filetype. However, some users do not use (or like) this functionality.
1. Open Outlook.
2. Go to the top of the Ribbon and click the Office Button.
3. Click Options.
4. In the left pane, select Trust Center.
5. In the right pane, click the Trust Center Settings button.
6. In the left pane, select Attachment Handling.
7. In the right pane, under Attachment and Document Preview, check the Turn off Attachment Preview checkbox. If you only want to disable Attachment Preview for certain types of files, leave the checkbox unchecked and click the Attachment and Document Previewers button. From the File Previewing Options window, uncheck the previewers that you do not want to use.
8. Click OK to close the window(s).