These files are made for Internet use. They are used as condensed files that can be accessed from websites. The main reason for that is economy on traffic or bandwidth and also web space for webmasters. And most importantly it saves the download time for the users accessing these files. However such files cannot be used for other purposes and if you convert them to other formats, the quality will suffer. But you can try to rename the extensions for the files that would not open in the software you use.