Thanks for the detailed feedback. As for file extensions, extensions are not actually a required thing in filenames, they're more of a loose convention. Websites tend to forego them depending on how their images are loaded (e.g. from data streams rather than files). It's the browser's responsibility to determine an extension if at all possible, based on data from the webserver if provided (such as a suggested filename, with or without extension). It's really not within an addon's power to ensure the presence of file extensions, or infer any, sorry.