limit file attachment types

Ability to restrict the types of files accepted. e.g. PDF only. or no ZIP's as a product attribute. For one type of work we only take PDF's, for some we need a zip (variable data) and I don't want a PDF or a .PS. Some nut tried to send me TIFF-IT's the other day and there was no way to reject them based on file type before they put in the order... painful conversation.