Docs for IS_IMAGE

    Checks if file uploaded through file input was saved in one of selected
    image formats and has dimensions (width and height) within given boundaries.

    Does *not* check for maximum file size (use IS_LENGTH for that). Returns
    validation failure if no data was uploaded.

    Supported file formats: BMP, GIF, JPEG, PNG.

    Code parts taken from
    http://mail.python.org/pipermail/python-list/2007-June/617126.html

    Args:
        extensions: iterable containing allowed *lowercase* image file extensions
        ('jpg' extension of uploaded file counts as 'jpeg')
        maxsize: iterable containing maximum width and height of the image
        minsize: iterable containing minimum width and height of the image

    Use (-1, -1) as minsize to pass image size check.

    Examples:
        Check if uploaded file is in any of supported image formats:

            INPUT(_type='file', _name='name', requires=IS_IMAGE())

        Check if uploaded file is either JPEG or PNG:

            INPUT(_type='file', _name='name',
                requires=IS_IMAGE(extensions=('jpeg', 'png')))

        Check if uploaded file is PNG with maximum size of 200x200 pixels:

            INPUT(_type='file', _name='name',
                requires=IS_IMAGE(extensions=('png'), maxsize=(200, 200)))