why do some illustrator files save as capital letters extension

why do some illustrator and other adobe program files save as capital letters extensions? Typically they save as .ai, .psd, .ind, etc, but sometimes I see them as .AI, .IND, .PSD etc and with no thumbnail preview. The lowercase files display thumbnail previews with the correct Widnows image codecs but none of them display previews for the uppercase extensions, even file viewers such as Faststone and xnview.