Source images without an extension in the human-readable filename get their cropped images saved incorrectly

If an image has been edited so its human-readable filename does not actually look like a filename (e.g. "Some Description" rather than "Some_Description.jpg") then True Crop saves the cropped image without an image extension in the file URI either, causing it to break.

The code should ensure that the destination image URI always has the same file extension as the source image URI.