Files uploaded/shared from Outlook Add-In loses extension when "Hide extensions of known file types" is enabled on the local machine.
Following workaround can be applied to fix this issue:
- Start Windows explorer.
- Click "Organize" drop down menu from the top left hand corner.
- Click Folder Options and then click "View" tab.
- Uncheck "Hide extensions of known file types"
- Click Apply and then click OK.