In Microsoft Office Outlook (including Outlook 2007, Outlook 2010, Outlook 2013, Outlook 2016 and etc.), users can preview supported files that sent to them as email attachments directly in Reading Pane (aka Preview Pane) or Message window without actually need to open or save the attachments to local disk. The file formats that can be previewed including Office Word, Excel, PowerPoint files, JPG or JPEG photos, GIF images, Acrobat PDF documents (instead of loading the PDF in bloated Adobe Reader installed) etc., and even supports viewing of ZIP file content when additional preview handler is installed.

When reading an email that attached with an attachment, there will be additional tabs displayed on top of the Reading Pane or Message window to indicate the availability of attachments. When you click on the attachment tab with a supported file format or extension, you will get a warning message about the security risk on previewing on the file from non-trustworthy source, and possibility that preview does not show full content of the file:

Outlook Previewing

At the end of the message, there is a check box for Always warn before previewing this type of file, which is checked (ticked) by default. You can deselect (untick) the check mark so that Outlook will automatically show the preview of the same file type of attachment in future.

However once you uncheck the option, Outlook 2007 will auto open and display the preview of attachment immediately, without going through the intermediary page again. So it’s impossible to check or enable again the warning prompt before previewing from the user interface within Outlook itself.

To re-enable the You should only files from a trustworthy source warning prompt and reset the Always warn before previewing this type of file option to original default, we have to use Registry Editor to modify the registry, in all Windows version including Windows XP and Windows Vista.

  1. Click on Start button.
  2. In Windows Vista, Windows 7, Windows 8, Windows 8.1, Windows 10 or newer, type RegEdit in Start Search bar, and click or tap on Registry Editor in the search result.

    For Windows XP and other operating system, click on Run command and then type RegEdit into Run text box, and hit Enter.

  3. If prompted, allow Registry Editor permission to run as administrator.
  4. In Registry Editor, navigate to the following registry key:


  5. Expand the Previewers registry key to see more subkeys.
  6. The sub-keys are named by on the file extension, and each of them contains a value named PONTState with value data set to 1. Locate the subkey for the file extension that you intend to reset the auto-preview behavior to the default ask-first-before-preview.
  7. Delete the subkey folder completely.

    For example, if you want to reset and disable PDF auto-preview, right click on pdf sub-registry key under Previewers, and then select Delete. Confirm key delete prompt.

Images and photos such as files in PNG, JPG or GIF file extension always preview automatically, and you cannot set Outlook to always ask before opening pictures, but you can disable the image preview handler.

The next time you view an attachment within Outlook, you will prompt with the intermediary page to ask for your confirmation again.