After an image has been successfully uploaded to your account, you can indeed change the extension part of the URL to be delivered with the image format that you need.
However, when using Cloudinary's 'Auto-upload' feature, to fetch the remote image for the first time, Cloudinary is looking for the original image as it was specified in the URL.
For example: If the original image is a png, and you fetch them by changing the extension to .jpg then it will break, as it is looking for the original URL which has .png extension.
The solution for this case is to set
fetch_format parameter to
f_jpg in URLs) to automatically upload the original JPG while delivering it as PNG.
Original Image : http://my-remote-website/original_png_image.png
When uploading to Cloudinary the fetch format can be set to jpg: http://res.cloudinary.com/demo/image/upload/f_jpg/folder1/original_png_image.png
Once the image is uploaded to Cloudinary, the extension can be changed without using fetch format.