determine image-type WITHOUT reading header information

Is there a way to determine the file type by directly scanning the file itself, rather than reading it from the header? I know it seems like overkill…but I don’t want someone to be able to edit the mime-type of a file (say changing it from “image/png” to “image/gif”) and then do the same thing with the file extension, without me being able to tell.

I experimented with this by using the ImageMagick method [B]setImageFormat[/B] to change a jpeg to a gif. Then I saved it to the harddrive with [B]writeImage[/B] and retrieved it again using GD. Then I used the [B]imagecreatefromgif[/B] function on it, and it returned true, even though it was still really a jpeg.

Is there any kind of test that will return false if the file contents don’t agree with the mime-type?

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