Test if uploaded file is JPEG, PNG or TIFF

I've been looking at some of uploads that went wrong on YayArt lately, and it turns out that people sometimes submit images with the wrong extension, e.g. "someimage.png" when it's really a JPEG. This confuses the image backend we're using to process large images, VIPS, so it reports back an error.

I did a bit of googling, and it seems the easiest way out is to simply check the first few bytes of the file for magic numbers. So here's a bit of Python code for checking for whether the file data belongs to a JPEG, PNG or TIFF image:

def is_jpg(data):
    return data[:2] == '\xff\xd8'

def is_png(data):
    return data[:8] == '\x89PNG\x0d\x0a\x1a\x0a'

def is_tiff(data):
    return data[:4] == 'MM\x00\x2a' or data[:4] == 'II\x2a\x00'

If the file is already on disk, you can grab the first few bytes with

f = open("somefile.jpg", 'r')
data = f.read(11)
if is_jpeg(data):
    ext = ".jpg"
elif is_png(data):
    ext = ".jpg"

Of course this won't test that the whole file is valid. But it's easier to do that afterwards with an image library once the extension is correct.

The magic numbers are documented in the specifications for the formats. You can also find some help for other formats in the source code of the file command on Unix systems.

Update: I'm liking this so much that I ended up putting it in a separate file and making a convenience function for getting an extension like '.jpg'. Grab the Python file here. I also added support for GIF. Here's another easy reference for magic file numbers.

Second update: I've updated the code, there was a bug detecting JPEGs from certain digital cameras that put Exif data in the first segment. Suffice to check the two first bytes of the JPEG, then the problem does not occur.