SQL Server seems to be ignoring specified backup file extension I have created a maintenance plan using the Wizard and this is the SQL produced, per SSMS:

BACKUP DATABASE [FOO] TO  DISK = N'C:\SQLBAKS\FOO\FOO_backup_2019_08_09_161656_6414445.diff'  
WITH DIFFERENTIAL , NOFORMAT, NOINIT,  
NAME = N'FOO_backup_2019_08_09_161656_6414445', 
SKIP, REWIND, NOUNLOAD, 
STATS = 10


The resulting files do not have a ".diff" extension but ".bak"

I am trying to differentiate FULL backups from DIFFERENTIALS in the filename.
When you go through the wizard you should


Click the differential option and 
Specify diff in the Backup file extension on the Destination Tab
(it's bak by default, even for differential)


Most people aren't fans of maintenance plans since you are kind of blindly turning over
some control and they create SSIS packages which aren't fun to troubleshoot. Ola's scripts are super easy, well documented, and have a ton of options. You can always roll your own via Agent Jobs too.