Hello,
Friendly URL extension - Specifies the extension of friendly URLs. The extension should be preceded by a dot when entered, such as: .aspx or .html. When you omit this value, the friendly URLs will be the same as the alias path (e.g. /products/nokia).
Files friendly URL extension - Specifies the extension for files that will be used in friendly URLs, such as ".aspx" or ".html". An example would be: getattachment/<node alias>/myimage.aspx
You should be able to leave both these setting blank and still return an image. Have you also configured your site for extensionless URL's per our documentation -
http://devnet.kentico.com/docs/devguide/configuration_of_extensionless_urls.htmBest Regards,
Edward Hillard