Is there a way to configure SSMS so that the backup file names for differential backups (created using the Maintenance Plan Wizard) would have the string "DIFF" in the name?
I would recommend using Ola's scripts.
Not only are they awesome but they will give you the naming convention that you are looking for.
You can customize the file extension on the Destination tab of the "Define Back Up Database (Differential) Task" step in the wizard. You could use
This will result in a maintenance plan that will execute a line something like this:
BACKUP DATABASE [Foo] TO DISK = N'K:\MSSQL\Backup\Foo_backup_2019_08_09_163546_1455200.diff.bak' WITH DIFFERENTIAL ...