I believe TIFF and TIF are exactly the same. Simply rename to *.tif
The problem is that your linked TIFF file are 24bit. Convert them to 8bit (reduce colours to 256) and OziExplorer is quite happy. Even the free program IrfanView converted 400000-rgb.tiff within 1 second.
PhotoShop will handle larger images - but 12GB is going to be difficult and probably very slooooow.

Mods is this a little OT. Should it be moved?

Perhaps a commmand line utility may be a little less memory hungry?
I just downloaded a free program called ImageMagick. Created a batch file (1.bat) containing:
convert 400000-rgb.tif +dither -colors 256 400000-rgb_256.tif
pause

Converted the TIF perfectly. Be prepared to wait hours for a 12GB effort. )-: