The imghdr module

This module identifies different image file formats. The current version identifies bmp, gif, jpeg, pbm, pgm, png, ppm, rast (Sun raster), rgb (SGI), tiff, and xbm images.

# File: imghdr-example-1.py

import imghdr

result = imghdr.what("samples/sample.jpg")

if result:
    print "file format:", result
else:
    print "cannot identify file"

file format: jpeg

import Image

im = Image.open("samples/sample.jpg")
print im.format, im.mode, im.size