Is HttpPostedFile.ContentType a flawless way to validate an uploaded file?

Question

I want to validate the file type to make sure the user is uploading an image of type JPEG, GIF, or PNG. Instead of checking the file extension, I figured using HttpPostedFil

Answer 1

Using the extension is probably safer. The ContentType is sent in the http request from the client. If you test for the extension, the user can change the extension of an exe to jpg, but it won't run as an exe.