In the inbox, the problem may be with the attachments themselves. A simple way to fix Outlook attachment issue is to convert the file into a.zip file and send it through Outlook. This will change the file extension to a '.zip' file. In addition to the.zip extension, the.zip file will reduce the size of the original. The original file will be included in the zip-file.

See also - https://emailspedia.com/unable-to-open-outlook-attachments/