It's not clear what are you trying to do. Where are you trying to upload these files? Is it in Media Library or what interface? Or is it CSS Stylesheets application (and by design it's for CSS only I guess)?
Typically we put these files somewhere into App_Themes folder and reference these files directly in layout. And not uploading those files via CMS interface, but just copy to file system (typically it is a part of automated rollout with continuous integration).