File upload checkout attribute

I've assigned a file upload control as a checkout attribute for our version 2.8 store.

Is there a way to allow only certain file extensions?  I tried catalogsettings.fileuploadallowedextensions under configuration --> all setings, but that doesn't seem to control the file upload checkout attribute.  I also can't seem to limit the file size.

Thanks.

This feature was first implemented in v3.3. Check the changeset