There is some limitation in any OS to length of extension?
I've searched "bdr" is from Microsoft
That's why I use long file extensions =)
Of course maybe some short extension like .skin is also used often and has 4 characters..
I think that I will keep zip, and in each application I use the zipped skins I will change the file extension. That's because if I want to do a file association it will cause problems (i.e user saying 'I can't install the skin').
Also if I put a general use file extension, i need to document that and i don't want to do it =)
If someone want to improve the format inside the zip (with other bitmap names, other format) skins will be not compatible ones with others.