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.

Start by reading the decoder-selection discussion linked in the issue, then try extensionless images in the app to observe whether browser MIME sniffing identifies them. Trace the existing decoder-selection mechanism and define the work as complete when the original image type is shown consistently, including pasted images.

Written by the indexing model from the issue text.