Nigger, do you seriously think that scanning the first few bytes of every single file in a directory to determine a magic number is a good idea? Like, there's a folder full of a few thousand .JPEG photos synced from your phone, you think that every single .JPEG needs to be checked to make sure that they do, indeed, look like JPEG files?
I'm sure that there's a bunch of special cases in KDE and that shitty alternative GUI for inferring MIME types when an extension is missing. But the sensible choice is just... having an extension. Or using a serious filesystem where the application responsible for 'owning' a file is stored in the resource fork.