Hi,
As my understanding there is no stadatrd way to validate digital sign document during upload.
Can you please clarify (when user tries to upload a document through my application) Do you mean you are attaching documents from other application and storing in DMS, If so you must require a validation in 3rd party application.Then upload to SAP throgjht standard BAPI.
Here in SAP you can control users with auth object for upload.
Rgds,
Nayeem.