check if an image is already a jpeg (or png?) and tell users that

See https://github.com/catdad-experiments/heic-convert/issues/22 and probably #7 as well.

iPhones, Google Photos, and various other apps are really bad about this. They'll often pre-convert it for you but keep the `.heic` file extension, so folks are left thinking the file is a heic when it is in fact not. This app won't convert it (as there is nothing to convert), which just confuses users.

We should add a message to tell them their image is not what they think it is.