One question: What extension do you use for your images: *.jpg or *.JPG or *.JPEG?
I say this because we made a change in the way we do MIME resolution. I have a nagging feeling that it is identified as an image named image.JPG, but it fails to resolve the converter (because that use the filename’s extension).
I will create an issue about this and figure out a proper fix, but in the meantime the workaround is to make sure that your images are named sunset.jpg, logo.png (e.g. lower case extensions).