Please follow below steps to prevent Outlook from always asking when opening attachments in Outlook.

1. Close your Microsoft Outlook.

2. Restart your Microsoft Outlook as administer. Click to know more about running Outlook as admin.
In Windows 10, please click the Search button  in the Task Bar, type Outlook into Search box, and then click the Outlook program with holding the Ctrl + Shift keys. See screenshot:

3. In the popping up User Account Control dialog box, please click the Yes button to go ahead.

Now you get into the Microsoft Outlook as administer.

4. In the Mail view, open the mail folder containing the email with the specified type of attachments, open the email in Reading Pane, and then double click the specified type of attachment in the Attachment Bar. See screenshot:

5. In the popping up Opening Mail Attachment dialog box, please uncheck the Always ask before opening this type of file option, and click the Open button. See screenshot:

6. Restart the Microsoft Outlook as normal.

From now one, the specified type of attachments will open directly when you double click them in the Attachment Bar, and the Opening Mail Attachment dialog box won’t come out never.

Note: If you have enabled the Protect View for Outlook attachments (File > Options > Trust Center > Trust Center Settings > Protected View > Enable Protected View for Outlook Attachments) in corresponding Microsoft Office programs, says Microsoft Word, the Opening Mail Attachment dialog box won’t come out too. However, the attachments will be opened in Protected View.

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