How to restrict file types or certain file extensions for RTC Work Item Attachment

We are currently using CLM version 6.0.1. We would like to restrict people from attaching arbitrary files to work items. On when hand we would like people to upload attachments on the other we would like to restrict it to certain extensions like .doc, .ppt, .pdf, .xls, xlsx etc. only. In principle we would like to block people from uploading files with extensions not in our whitelist.

I have written some basic RTC extension for custom notification earlier and I would like to know if this approach is possible. We can create an Operation Advisor which will get the file extension of all the attachments of the work Item and prevent save, if the file type is not in our whitelist. Any sample code to get the attachment file name, URL etc will help a lot.