The Central Upload Agent with Content scanner will not assign the correct user to the file. It will give the information of who is the most probably owner of the file (common sender, common receiver etc.).
Files are assigned to the account that discovered it, which in this case is MMPST Service account.
Content scanner will populate columns: suggested owner, common sender, common receiver etc. on the Migration manager console > Owners screen.
The Central Upload Agent has nothing to do with scanning of the content of files, it's only uploading the files. Content scanner is a separate module from the CUA agent.