Surprisingly we don’t have much documentation about it but try these two properties of Output Variable.
LocalPath
FullName
As per my understanding, the activity downloads the file to some temp folder like our Outlook attachments gets stored.
We will have to explicitly copy the file from that Temp folder to desired location. Sorry, I don’t have O365 integration to validate this but one of these property should have the local path of the file.
Thanks,
Ashok 