Adjusts the contrast of image files (PNG, JPG, TIFF and other formats supported
by Pillow). The factor controls the strength: 0.0 produces a solid grey
image, 1.0 leaves the image unchanged, and values above 1.0 increase
contrast (a useful range is roughly 0.0–2.0). A factor of 0.5, for
example, reduces contrast by about 50%.
Files that are not images are passed through unchanged.