files = ["img12.png", "img10.png", "img02.png", "img1.png", "IMG01.GIF"]

result = sorted(files, key=lambda f: (
    __import__('re').match(r'([a-zA-Z\-_.]+)(\d+)', f).group(1).lower(),
    int(__import__('re').match(r'([a-zA-Z\-_.]+)(\d+)', f).group(2))
))

print(result)