When pdf files have images created from photoshop or adobe ai in them, file-type detects them as .ai based on the byte checking heuristic we have in place.

I'm proposing that even if the magic string is found, if the original file's extension is .pdf, file-type should consider it a pdf and not change it's type based on some content inside of it.

An even more strict approach that I would also support is only returning ai file type if the file extension is already .ai. It seems more natural/compatible to default to .pdf if .ai isn't explicitly specified, since the ai detection is just a loose heuristic anyway.