Purpose : Overrides to change the maximum file size uploaded via the Drag/Drop Attachments control or through select files.. if user tries to attach a file of size more than this, UI error is shown. Usually it is best practice to specify a size which is less or equal to initialization/maximumfileuploadsizemb value in prconfig. This is to ensure both values are in sync .
Alternate Option [ older version of pega ]:
You may try adding the same setting in DSS as below.