Adjusts the brightness of image files (PNG, JPG, TIFF and other formats supported
by Pillow). The factor controls the strength: 0.0 produces a completely
black image, 1.0 leaves the image unchanged, and values above 1.0 increase
brightness (a useful range is roughly 0.0–2.0). A factor of 1.2, for
example, brightens the image by about 20%.
Files that are not images are passed through unchanged.