Plugin or hack to verify/restrict user file upload by type?

Is there an available plugin or hack to verify and restrict the types and sizes a standard WP user role can upload?

All of the threads on this subject I’ve found seem to have died unanswered, and a plugin I found which supposedly does this can easily be defeated by changing the file extension.

Surely there’s something which solves this obvious need, but I must be just missing it? Any pointers to an elegant solution?