It seems that you can try also using a format control query of the SYSATTACHMENT table which is the validated by a status dependent validation. A further suggestion of validating the uploader of the attachment was also suggested as a possibility.
This isn’t available out of the box but you should be able to tailor something to suit your needs. You didn’t mention which SM records you are working with so I will assume it is Incidents, although the principle would be the same for any.
You should be able to make a query from the Queries section of the probsummary format control to the SYSATTACHMENTS table to check that it contains records where the topic of the SYSATTACHMENTS record equal the number of the Incident record. This can then be used in the Validations of the format control record using not null(contents($file#)) where # is the actual file query number from the format control query. So if it was the third query (from top to bottom) on the format control queries page then the file variable would be $file3.