If you can find out the same type of files (the CSV file in my example) in your Computer, please follow below steps to change the default programs or file associations about opening specified type of Outlook attachments.

1. Go to the folder containing the specified type of file, right click this type of file, and select Open with > Choose another app (or Choose default program) from the right-clicking menu. See screenshot:

2. In the opening dialog box, please: (1)Click to select the new program you will open this type of attachments with; (2) Check the Always use this app to open .CSV files option (or Always use the selected program to open this kind of file option); (3) Click the OK button.

3. Now the selected file is opening with the specified program. Please close it without saving.

From now on, all files of the specified type will be opened with the new program by default, including this type of Outlook attachments.

Quickly export and save all attachments from multiple selected emails in Outlook

Normally we can save attachments from one email with activating the Attachment Tools and applying the Save All Attachments feature in Outlook. But, what if saving attachments from multiple emails, or from the whole mail folder in Outlook? Try Kutools for Outlook's Detach All (Attachments) feature.

By the way, Kutools for Outlook also provides a workaround to automatically save or export all attachments from all incoming emails in Outlook.

You can also associate the specified file type of Outlook attachments to a program in Control Panel to change the default programs of opening specific kind of Outlook attachments.

1. Please open the Default Programs in Control Panel with opening a folder and pasting below path into the address box:
Control Panel\All Control Panel Items\Default Programs

2. Now you get into the Default Programs dialog box, please click the Associate a file type or protocol with a program option. See screenshot:

3. In the Set Associations dialog box, please click to select the extension of the specified file type (.csv in my example), and then click the Change program button. See screenshot:

4. Now in the popping up dialog box, please click to select the new program you will open the specified type of files with, and click the OK button. And finally close the Set Association dialog box. See screenshot above:

From now on, all files of the specified file will be opened with the new program by default, including this type of Outlook attachments.

Change the default picture viewer in Outlook

Switch/change default profile in Outlook

Start Outlook with all reading panes off by default