How to block/Prevent certain file formatted files to get uploaded into Windchill ?

According to CS109414, this functionality doesn't exist at least in WC 10.1, and I doubt it exists in any later function as well.  You would need to deploy a customization to add a "checker" on the file extension by modifying the PTC create WTDocument code to filter out the formats to be blocked and then display en error to the user.

What is your use-case for this functionality?