Mělo by to jít jednoduše (nastavení asociace). Mrkni:
https://support.microsoft.com/en-us/office/change-the-file-association-for-an-email-attachment-6fa9ac47-4e57-4ab8-9234-2422e3ea507f
https://www.msoutlook.info/question/706
https://www.extendoffice.com/documents/outlook/3818-outlook-attachment-default-program-file.html