Temporary Attachment Storage

If you open, edit, and save an attachment in Outlook 2016+ without saving it to a common location on your local drive (i.e. desktop), or to a network share, the file is stored in the default temporary SecureTemp folder for Outlook.  

To open the folder where the SecureTemp folder Outlook is using is stored, type or paste the following into the address bar of File Explorer and press Enter.

C:\Users\%username%\AppData\Local\Microsoft\Windows\Temporary Internet Files\Content.Outlook