Security - Restrict File Types Allowed For Upload?

I want to limit the allowed uploaded file types to images, pdfs, and docs. What is the recommended way to approach this?I assume checking the file extension alone is not enough, since an attacked can change the file extension as he wishes.I also thought about checking against MIME Type using PostedFile.ContentType.I still don't know if this is adding any further functionality than checking against file extensions alone, and if an attacker have and ability to change this information easily.This is basically for a course management system for students to upload assignments and teachers to download and view them.