scan folder for .jpg files that are really GIFs

import os
import sys
import glob

GIF_TAG = 'GIF'

try:
    FOLDER = sys.argv[1]
except:
    FOLDER = '.'    # default to current directory

for path in glob.glob(os.path.join(FOLDER, '*.jpg')):
    magic = open(path, 'rb').read(len(GIF_TAG))
    if magic == GIF_TAG:
        print 'File is really GIF', path