How to disable certain file types from previewing in the Outlook Preview Pane

To disable a certain file type from being previewed within the Outlook 2013 Preview Pane, access Options –> Trust Center –> Trust Center Settings button.

Access Attachment Handling –> Attachment and Document Previewers button.

Find the file type in question (RTF in this example) and remove the checkmark.  Click on the OK button.