Hi,
Thank you for posting in Microsoft Community, we appreciate your interest in Windows 10.
I understand the inconvenience that you are facing on Windows 10, we will certainly help you with this issue.
In order to assist you better, I would like to know the following queries:
Are you able to download attachments through web browser?
Does the issue occur only with outlook.com mails?
Meanwhile I suggest you to run the app troubleshooter and check if it helps**.**
**http://windows.microsoft.com/en-us/windows-10/run-the-troubleshooter-for-windows-apps**
Hope this helps. Please respond if you have any further related query.
Thank you.