When showing 'original', indicate the type

In some cases, the type of the original isn't clear to the user, especially when it comes to pasting an image. The type of the original should be indicated. We should do this by the same mechanism we use to decide which decoder to use (https://github.com/GoogleChromeLabs/squoosh/pull/91#discussion_r201663191). For formats with broad browser support, I wonder if the mimetype is already populated by sniffing. We should try throwing some extensionless images at the app and see what happens.