Do we have an efficient way to give warning during user upload the document in Appian?
when he press "Upload" in file upload object, choose a highly restricted doc and open, then system can give a warning Popup show this is highly restricted doc, are you sure you want to upload? choose confirm then upload, cancel then purge the doc directly.
Discussion posts and replies are publicly visible
To add to what others here have already suggested - the only thing slightly stronger than validating against a certain file extension is to enable File Extension Blocking in the Admin Console. Again this only has to do with file extensions and that doesn't necessarily correlate to "highly restricted" per se.