Open the image in your browser (I'm using Chrome, windows).
1. Drag the image to your desktop -> you'll get a png image rather than a jpg. If you'll right-click to <Save image as..> it will save as jpg.
2. Right-click to <Save image as..> to get the jpg image. Try to upload it as an attachment to your post, you'll get an error "The contents of the uploaded image do not match the file's extension."