I'd also check if you can configure Tomcat or the servlet container you're using to reject uploads by mime-type or file extension.
It may be simple, and if it succeeds, it should be fairly robust.
Under Admin -> Attachments, set the maximum file size to something that is ok for documents, but useless for video. This won't stop it, but your users won't be able to upload videos of much use.
If that's not going to work for you, then you need add-ons that can scan and refuse attachments - there's a couple in the marketplace, have a look for them.
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