Files uploaded or shared from the 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: 

  1. Start Windows explorer.
  2. Click the Organize drop down menu from the top left hand corner. 
  3. Click Folder Options.
  4. Click the View tab. 
  5. Uncheck Hide extensions of known file types.
  6. Click Apply and then click OK.