Hello,

The extension is taken from Files friendly URL extension property, which can be found in the Site Manager -> Settings -> URLs and SEO. So if you have set there an .aspx extension, the image editor add this extension to the URL address of that uploaded image, but this is only for a SEO optimalization, your jpg image is also aviable on the URL with the original extension (e.g. ~/Images/Picture.aspx and also ~/Images/Picture.jpg)

Best regards,
Jan Hermann