By design "non standard" tiff files are skipped when loading.
12-bit (RGB 121212) tiff files however are not that uncommon and both PS and Windows image viewer loads them successfully.
The most common source for such images is specialized (photographic) equipment.
Enabling this is not that hard - we just expand them to 16bit, but is not trivial either - no ready-made routines.
(attachment to big)