How to check if an uploaded file is an image without mime type?

Question

I'd like to check if an uploaded file is an image file (e.g png, jpg, jpeg, gif, bmp) or another file. The problem is that I'm using Uploadify to upload the files, which changes the mime type and gives a 'text/octal' or something as the mime type, no matter which file type you upload.

Is there a way to check if the uploaded file is an image apart from checking the file extension using PHP?

Answer 1

My thought about the subject is simple: all uploaded images are evil.

And not only because they can contain malicious codes, but particularly because of meta-tags. I'm aware about crawlers that browse the web to find some protected images using their hidden meta-tags, and then play with their copyright. Perhaps a bit paranoid, but as user-uploaded images are out of control over copyright issues, I take it seriousely into account.

To get rid of those issues, I systematically convert all uploaded images to png using gd. This have a lot of advantages: image is clean from eventual malicious codes and meta tags, I only have one format for all uploaded images, I can adjust the image size to fit with my standard, and... I immediately know if the image is valid or not! If the image can't be opened for conversion (using imagecreatefromstring which doesn't care about image format), then I consider the image as invalid.

A simple implementation could look like this:

function imageUploaded($source, $target)
{
   // check for image size (see @DaveRandom's comment)
   $size = getimagesize($source);
   if ($size === false) {
      throw new Exception("{$source}: Invalid image.");
   }
   if ($size[0] > 2000 || $size[1] > 2000) {
      throw new Exception("{$source}: Too large.");
   }

   // loads it and convert it to png
   $sourceImg = @imagecreatefromstring(@file_get_contents($source));
   if ($sourceImg === false) {
      throw new Exception("{$source}: Invalid image.");
   }
   $width = imagesx($sourceImg);
   $height = imagesy($sourceImg);
   $targetImg = imagecreatetruecolor($width, $height);
   imagecopy($targetImg, $sourceImg, 0, 0, 0, 0, $width, $height);
   imagedestroy($sourceImg);
   imagepng($targetImg, $target);
   imagedestroy($targetImg);
}

To test it:

header('Content-type: image/png');
imageUploaded('http://www.dogsdata.com/wp-content/uploads/2012/03/Companion-Yellow-dog.jpg', 'php://output');

This does not exactly answer your question as this is the same kind of hack than the accepted answer, but I give you my reasons to use it, at least :-)