If you check the box to always open an attachment in the Open or Save dialog and later decide to change the behavior, you need to edit the file type in Windows Explorer.

1. Open Windows Explorer and go to its Tools, Folder Options, File Types.
2. Select the file type from the list. (Tip: the columns are sortable.)
3. Click the Advanced button.
4. Add a check to the box to Confirm open after download at the bottom of the dialog.

Note that if you have a document type set to open, malicious code can be used with some versions of Outlook to automatically open those attachments types.