2006.07.24 03:49 "Re: [Tiff] BigTIFF Implementations", by Bob Friesenhahn
File extensions are for humans. File formats which support accurate identification based on their header don't need to rely on a file extension at all so it could be left off, or be some random value, and properly written applications should still be able to open it.
Several different extensions are already used for TIFF files so use whatever causes the least confusion for your users.
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/