Provide a hint when an image can't be decoded, but it is a valid image

[alexwlchan]

I've just worked on a collection of images which dominant_colours refused to look at, because they were named with JPEG file extension but were actually WebP images. Here's a simple repro:

$ cp src/tests/purple.webp src/tests/purple.jpg

$ dominant_colours src/tests/purple.jpg
Format error decoding Jpeg: Error parsing image. Illegal start bytes:5249

$ file src/tests/purple.jpg
src/tests/purple.jpg: RIFF (little-endian) data, Web/P image, VP8 encoding, 1x1, Scaling: [none]x[none], YUV color, decoders should clamp

Under the hood, dominant_colours is using Rust's image crate which relies on file extensions.

In this project, I was able to change the file extensions to match the image type, but I had to know to run file to identify that these were really valid images.

It would be useful if dominant_colours could suggest that a thing that looks like a JPEG image is actually a WebP image, or some other error message that points in the right direction.